The White House says the situation in Ukraine will be "front and center" during President Barack Obama's trip to Europe next week.
National Security Advisor Susan Rice told reporters Friday that the common theme to the president's trip is the fundamental strength of U.S. partnerships and alliances, including NATO, the European Union and the G7.
Rice said Ukraine and the Russian takeover of Crimea are prompting a fundamental reassessment of U.S.-Russian relations. She said the world will clearly see that Russia is more and more isolated.
Deputy National Security Advisor Ben Rhodes said a G-7 summit in The Hague -- a meeting that probably would have included Russia as an eighth member -- has been added to the president's agenda as part of that isolation.
Also on the president's European schedule is a nuclear security summit with more than 50 other countries, including Russia.
Rice said the United States has every interest in continuing to cooperate with Russia on this issue, which she calls a pillar of the Obama national security policy -- making it harder for terrorists to get their hands on nuclear materials.
Monitors en route to Ukraine
Meanwhile, Russia and the rest of the OSCE [the Organization for Security and Cooperation in Europe] agreed Friday to deploy civilian monitors to Ukraine, but there is no mention if they will go to Crimea.
The OSCE, which works by consensus, says up to 500 monitors will gather information on the security situation in Ukraine, including human rights.
The United States said the OSCE has a mandate to work in all of Ukraine, including Crimea. But Russia's ambassador to the OSCE, Andrey Kelin, said Crimea is part of Russia and the mission has no mandate there.
Russia has thwarted several efforts to deploy monitors in Crimea. Pro-Russian forces stopped OSCE military observers from crossing into Crimea last week.
Watch related video by VOA's Henry Ridgwell
Putin signs law annexing Crimea
Russian President Vladimir Putin Friday signed legislation completing the annexation of Crimea, while Ukraine initialed a deal for closer political cooperation with the European Union.
The Kremlin signing ceremony took place after the Federation Council, the upper house of Russia's parliament, voted unanimously to incorporate the Crimea region into Russia. The lower house ratified the treaty on Thursday, just four days after Crimean residents voted in a referendum to break away from Ukraine and join Russia.
President Putin and Crimean leaders signed the treaty on Tuesday, and the Kremlin said the treaty came into force on the date it was signed.
In Brussels, meanwhile, Ukraine's interim Prime Minister Arseniy Yatsenyuk signed a political association agreement with the European Union. Also Friday, the EU added 12 people - all Russian or Crimean - to a list of those subject to travel bans and asset freezes, raising the number of such individuals to 33.
Among those newly added to the EU sanctions list are Russian Deputy Prime Minister Dmitry Rogozin and Dmitry Kiselyov, the controversial TV anchor who heads the Russian government's Rossiya Segodnya news agency.
The United States has also imposed sanctions on Russian officials - including several members of President Putin's inner circle - and a prominent Russian bank.
President Barack Obama says he is considering more penalties against entire sectors of the Russian economy. He says those plans are being formulated in conjunction with European allies.
Retaliation on hold
In a meeting Friday with his advisory Security Council, Putin said for now, Russia will hold off on imposing sanctions on the United States in retaliation for U.S. sanctions against Russia over the Ukraine crisis.
However, his spokesman, Dmitry Peskov, told reporters separately that Russia will respond in kind to the latest U.S. sanctions, which target Russian officials and members of President Putin's inner circle. "We will respond every time," Peskov said.
And the Russian Foreign Ministry said in a statement Friday that Russia will "respond harshly" to the latest round of U.S. sanctions.
Warning from Britain
On Friday, British Prime Minister David Cameron warned the Kremlin that Russia would face far reaching consequences if it sent troops into Ukraine after a referendum in Crimea which he said has been carried out under the barrel of a Kalashnikov.
"The best rebuke to Russia is a strong and successful Ukraine,'' Cameron said, adding that if Russian troops went into eastern Ukraine then the Kremlin would face "far-reaching consequences in a broad range of economic areas.''
Cameron also said that Russia is more dependent on Europe than Europe is on Russia.
US ships food
The U.S. is planning to ship about 300,000 ready-to-eat meals to Ukrainian military forces, even though a broader billion-dollar aid package for Kyiv is stalled in Congress.
The Defense Department said Friday that the U.S. focus is on "non-lethal" aid in the aftermath of Russia's annexation of Ukraine's Crimean peninsula. Moscow has moved quickly this week to incorporate Crimea into the Russian landscape after its predominantly Russian-speaking population voted overwhelmingly last Sunday to join Russia.
A Defense spokesman said the 25,000 cases of food are in Europe but have yet to be shipped to Ukraine.
The U.S. has proposed $1 billion in aid for Ukraine, but the measure has stalled in Congress, where lawmakers have disagreed on an unrelated provision covering $60 billion in U.S. spending for the International Monetary Fund.
U.N. Secretary-General Ban Ki-moon met Friday with Ukraine's acting president, Oleksandr Turchynov, and acting Prime Minister Arseniy Yatsenyuk in Kyiv for discussions on resolving the crisis.
On Thursday, Ban met with Putin and Foreign Minister Sergei Lavrov in Moscow, where he urged the Russian leader to prevent "any unintended incident" in or near Ukraine that could aggravate tensions in the volatile region.
The office of the U.N. High Commissioner for Human Rights said Friday that Ban's assistant, Ivan Simonovic, is in Crimea. During his two-day visit, Simonovic will lay the groundwork for a U.N. human rights monitoring mission to set up a presence in Crimea the announcement said.
Images from Crimea
A Ukrainian soldier walks past Russian soldiers marching to their camp outside a military base in Perevalne, Crimea, March 21, 2014.
Uniformed men, believed to be Russian troops, march outside a military base in Perevalnoye, near the Crimean city of Simferopol, March 21, 2014.
Ukrainian Navy sailors raise the Ukrainian flag on the Ukrainian navy command ship Slavutych as it is blocked by two Russian ships at the Crimean port of Sevastopol, March 21, 2014.
Russian sailors walk near the Ukrainian ship Slavutich in Sevastopol, March 20, 2014.
The Ukrainian ship Slavutich is seen blocked by two Russian ships at the harbor in Sevastopol, March 20, 2014.
A Ukrainian soldier walks near a closed entrance gate at an airforce base in the Crimean town of Belbek, March 20, 2014.
Workers put up a new sign reading "State Council of the Crimean Republic" at the parliament building in Simferopol, March 19, 2014.
Armed men, believed to be Russian servicemen, walk on the territory of the naval headquarters in Sevastopol, Crimea, March 19, 2014.
A Ukrainian naval officer passes by armed men, believed to be Russian servicemen, as he leaves the naval headquarters in Sevastopol, March 19, 2014.
A member of a pro-Russian self-defense force takes down a Ukrainian Navy flag, left, as another raises the Russian flag at Ukrainian Navy headquarters in Sevastopol, Crimea, March 19, 2014.
On Thursday, President Barack Obama announced further sanctions against Russia. Speaking at the White House, Obama said he is penalizing more individuals and a bank involved in Russia's occupation of the Black Sea peninsula. Earlier, the United States limited travel and the economic activity of several key Russians involved in the Crimea takeover.
The president also said he is considering more penalties to entire sectors of the Russian economy, as well as specific individuals. He said those plans are being formulated in conjunction with European allies.
EU travel ban, asset freeze
In Brussels, meanwhile, European Union leaders added 12 people -- all Russian or Crimean -- to a list of those subject to travel bans and asset freezes. The new names were not immediately released, but the move raises the number of people subject to EU sanctions to 33.
Moscow retaliated against the U.S. sanctions with travel bans on nine Americans, including Republican Senator John McCain, Democratic Senate Majority Leader Harry Reid and Republican Speaker of the House of Representatives John Boehner.
On Wednesday, U.S. Defense Secretary Chuck Hagel spoke for nearly an hour with Russian Defense Minister Sergei Shoigu. A Pentagon spokesman says Hagel asked why Russian troops are deploying along Ukraine's eastern southern borders. The spokesman said Hagel was assured that the troops are conducting maneuvers and have no plans to enter Ukraine.
Ukrainian officials say armed pro-Russian forces seized three Ukrainian warships in Crimea's Sevastopol Bay Thursday.
Ukraine authorities said Tuesday that Kyiv has drawn up plans to evacuate Crimea, and will seek U.N. support in turning the peninsula into a demilitarized zone. Ukraine Defense Council Secretary Andriy Parubiy also said Ukraine is planning to hold military maneuvers "with our allies." He did not elaborate.