U.S. Secretary of State John Kerry has accused Russia of not taking "a single concrete step" to de-escalate the Ukrainian crisis.
In a blistering attack Thursday, Kerry said Russia has failed to uphold the four-party deal it signed last week to ease tensions in eastern Ukraine.
He said Moscow's refusal to restrain pro-Russian separatists there will be "an expensive mistake," saying more sanctions are possible.
On Friday, credit agency Standard and Poor's cut Russia's credit rating to BBB- , underscoring the risks of Russia's policies in Ukraine.
The agency said it is concerned about increased capital outflows from Russia, and said the rating could be cut further if sanctions are tightened.
U.S. officials say President Barack Obama, who is traveling in Asia, will call European leaders Friday to discuss further sanctions against Moscow.
Meanwhile, Russia is blaming Kyiv and Washington for stoking tensions by cracking down on armed pro-Russian separatists who are occupying buildings in eastern Ukraine.
Moscow's Foreign Ministry referred to Ukrainian authorities as "under the patronage of the United States." It also said the situation demands "urgent action" by Washington to ensure the withdrawal of Ukrainian forces from the region.
Separately, at the United Nations, diplomats say Russia has recalled its ambassador to Moscow for emergency consultations.
The flurry of diplomatic exchanges capped a day of rising tensions along the Ukraine-Russian border, where a huge Russian military force is gathered. A Ukrainian diplomat at the United Nations told VOA that Moscow has doubled its military presence on the border to about 80,000 troops.
Russia ordered that force into new maneuvers Thursday after fighting between Ukrainian security forces and armed pro-Russian separatists left several militants dead in the flashpoint eastern city of Slovyansk. Russian President Vladimir Putin called the Ukrainian push against the separatists a "crime."
Ukraine earlier this week relaunched what it is calling "anti-terrorist" operations against separatists in about a dozen eastern cities. The separatists are demanding a referendum on whether to split with Ukraine and join Russia.
U.S. President Obama, speaking to reporters earlier Thursday in Tokyo, said Russia is not abiding by last week's Geneva agreement. He said he is not hopeful that Moscow will cooperate to ease regional tensions. That deal, approved by envoys from Russia, Ukraine, the United States and the European Union was aimed at defusing the crisis.