BASEL, Switzerland – U.S. Secretary of State John Kerry said Thursday that hundreds of Russian soldiers had fought in Ukraine and that many had been killed there. He urged Moscow to carry out the peace agreement that was negotiated in Minsk, Belarus, in September.

Kerry's remarks were among the sharpest he has delivered on the crisis and were made shortly before he met with Sergei Lavrov, the Russian foreign minister.

"Regrettably, Russia continues to supply new weapons and increase support for armed separatists," Kerry said in Basel at a meeting of the Organization for Security and Cooperation in Europe, which has sent teams to Ukraine to monitor a shaky cease-fire. "In doing so, it fails to meet its international and OSCE obligations and to live up to an agreement that it actually negotiated and signed."

He added, "The result is damage to its credibility, and its own citizens wind up paying a steep economic and human price."

Russia has steadfastly refused to acknowledge that it has ordered its own forces into Ukraine or that its military has suffered significant casualties there. Kerry's comments appear to be a calculated effort to highlight this allegation. Lavrov declined to respond to the allegations, but asserted that the Kremlin was still committed to carrying out the Minsk agreement.

Kerry also met with his Ukrainian counterpart, Pavlo Klimkin. "We're still working on a real sustainable cease-fire," Klimkin said. "And now it's about Russia, to deliver."

Kerry said, "His answer stands."

New York Times