The Joe Biden administration is stepping up pressure on Russia over a buildup of troops along its frontier with Ukraine.

As Secretary of State Antony Blinken prepared to meet his Ukrainian counterpart in Washington on Wednesday, the top U.S. diplomat for Europe said the administration is watching the border situation closely and would regard any escalation with deep concern.

Karen Donfried, assistant secretary of state for Europe and Eurasia, told The Associated Press that senior Russian officials have been warned of potential consequences for any increased threat to Ukraine's security from what she called "unusual military activity near Ukraine." She would not discuss specific consequences for Russia, but administration officials have in the past said boosting military support for Ukraine is one option.

Donfried was part of a U.S. delegation led by CIA Director William Burns that visited Moscow last week and delivered that message personally to Kremlin officials. After leaving Moscow, Donfried traveled to Kyiv to brief Ukrainian officials on the meetings.

"Any time we see unusual Russian military activity near Ukraine we make clear that any escalatory or aggressive action is of great concern to the United States," she said. "We're very clear that we support Ukraine's sovereignty and territorial integrity and that our commitment to that has not changed and is unwavering and that we will continue to stand with Ukraine and we will condemn any Russian aggression against Ukraine in all its forms."

Speaking of the Moscow meetings, which officials have said included a conversation with Russian President Vladimir Putin and one of his most influential advisers, she said, "Director Burns was effective in sending the messages that he thought it was appropriate to send."

"The indication of how they processed the information that was shared? The 'proof will be in the pudding', as we say," Donfried said. "We will continue to watch very closely what the Russians are doing on Ukraine's border."

A day after Burns and Donfried visited Moscow, Ukraine complained that Russia has kept tens of thousands of troops not far from the countries' border after war games, as part of an attempt to exert pressure on its ex-Soviet neighbor.

Ukraine's Defense Ministry said last Wednesday that about 90,000 Russian troops are stationed not far from the border and in rebel-controlled areas in Ukraine's east. It said specifically that units of the Russian 41st army have remained in Yelnya, about 160 miles (260 kilometers) north of the Ukrainian border.

Russia has cast its weight behind a separatist insurgency in Ukraine's east that erupted shortly after Moscow's 2014 annexation of Ukraine's Crimean Peninsula and has left more than 14,000 people dead. Russia has repeatedly denied any presence of its troops in eastern Ukraine.

Earlier this year, a massive buildup of Russian troops in the Russia's west raised concern in Ukraine and in the West, fueling fears of an escalation of large-scale hostilities.

Russian officials said the troops were deployed for maneuvers, casting them as part of measures to counter security threats posed by the deployment of NATO forces near Russian borders. Russia and the alliance also have blamed each other for conducting destabilizing military exercises near the borders.

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Republished with permission of The Associated Press.