President Barack Obama has called on Russia to honor a September cease-fire between Kyiv and pro-Russian rebels in eastern Ukraine.
White House Deputy National Security Adviser Ben Rhodes said President Obama delivered the message to Russian Vladimir Putin during informal conversations at a summit this week in Beijing.
Rhodes also told reporters that Obama will discuss the Ukraine crisis with European leaders on the sidelines of next week's G20 summit in Brisbane, Australia.
NATO and European monitors have reported Russian tanks, troops and weapons crossing the border in the past few days.
Moscow has repeatedly denied any official military involvement in Ukraine, and has described Russian troops fighting alongside rebels as volunteers.
Kyiv pledges to uphold truce
Meanwhile, Kyiv says it has has no intention of abandoning the truce deal it signed with Moscow and rebels in early September in Minsk, a government spokesman said Thursday.
Armed personnel and military trucks without markings are seen near a checkpoint on territory controlled by the self-proclaimed Donetsk People's Republic in Donetsk, eastern Ukraine, Nov. 12, 2014.
“There is no intention of abandoning [the ceasefire] despite all... attempts to break these agreements by the Russian side,” National Security and Defense Council spokesman Andriy Lysenko told a news briefing in the Ukrainian capital.
There have been almost daily breaches of the two-month-old truce, with violence having increased in the past weeks. Both sides blame each other for the violations.
Lysenko said that a buildup of Russian military hardware is continuing in areas of eastern Ukraine controlled by pro-Moscow separatists, along with military forces along Russia’s border with Ukraine, including long-range air defense and aviation teams.
More than 4,000 people have been killed in the conflict since April, according to United Nations figures.
Among those killed, 1,052 were members of Ukrainian government forces, Lysenko said earlier this week, adding that 4,079 people have been wounded as a result of the fighting.
UNSC emergency session
At an emergency meeting of the U.N. Security Council Wednesday, Ukraine's ambassador to the world body said Kyiv's restraint was the only reason all-out war has not erupted. Yuriy Sergeyev also questioned whether the Kremlin was planning an armed invasion in eastern Ukraine.
FILE - U.S Ambassador to the U.N. Samantha Power
U.S. Ambassador Samantha Power said Russia and the separatists have not lived up to any of the conditions of the September 5 Minsk cease-fire agreement, including pulling out all foreign fighters from Ukraine.
Power said at the same time, Ukraine has made a genuine effort to stick to the deal, calling on the U.N. to tighten the pressure on Russia.
Russia dismisses charges as propaganda
Deputy Russian Ambassador Alexander Pankin told the council it is Ukraine that is concentrating its forces along the front lines after Kyiv and the separatists agreed to pull them back. He accused European monitors of failing to take note of this and called reports of a Russian military buildup propaganda.
The Ukraine conflict has severely strained ties between Russia and the West, with the U.S. and the EU imposing a series of sanctions against Moscow, with Russia retaliating with its own punitive measures.
On Thursday, Russian Prime Minister Dmitry Medvedev said it was up to the U.S. to improve relations.
"In order to overcome existing problems, sanctions need to be simply abandoned, relations should be returned into a [level] playing field; normal, peaceful, productive negotiations should be restarted, then everything will stabilize," said Medvedev.
Representatives of the EU and the U.S. are meeting in Brussels Thursday to discuss additional sanctions against Russia’s economy and separatists in Ukraine.
After meeting German Foreign Minister Frank-Walter Steinmeier in Berlin Thursday, U.S. Ambassador to the United Nations Samantha Power said she and Steinmeier were “deeply disturbed, particularly by the military escalation that is afoot.''
“In terms of next steps... our experts are meeting this week in order to see how we can jointly impose costs on Russia,'' she said.