Washington says Russia's continued support for attacks by separatists against government forces in eastern Ukraine is a violation of a fresh cease-fire agreement reached in Minsk this month and "is undermining international diplomacy and multilateral institutions" that are "the foundations of our modern global order."
U.S. State Department spokeswoman Jen Psaki also said the United States is "particularly concerned about new attacks near Mariupol" on the coast of the Sea of Azov by Russian-backed separatists.
Psaki made the criticisms on February 20, hours after European Union President Donald Tusk said he was consulting with EU leaders about new measures to pressure Russia and the separatists to fulfill their obligations under the fragile new cease-fire deal.
Tusk said there had been "more than 300 violations" of the latest Minsk cease-fire deal since it was meant to come into force on February 15.
Tusk said the "European Union has been devoting all efforts to make the Minsk-2 Agreement work, even in the face of continued ruthless attacks on Debaltseve and other regions by the separatists militarily supported by Russia."
He warned that the EU is "clearly reaching a point when further diplomatic efforts will be fruitless unless credibly backed up by further action."
Tusk said he would confer with European leaders in the coming days and visit Kyiv on February 22.
Earlier on February 20, similar concerns were expressed in Paris by French President Francois Hollande and German Chancellor Angela Merkel.
Hollande said: "The cease-fire has been violated several times. It must now be fully respected along the entire front line."
He also said any country that fails to respect the Minsk agreements would face sanctions.
Merkel, for her part, said the European Union "could raise the issue of sanctions" if the Minsk agreements are not implemented.
"We didn't go to Minsk to impose sanctions. We went to achieve peace, which is a long and difficult process. But it is important to make every effort," she said.
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Meanwhile, French Foreign Minister Laurent Fabius said he would meet with his Russian, German, and Ukrainian counterparts in Paris on March 3 to discuss Ukraine's conflict.
In Kyiv, Ukrainian army spokesman Andriy Lysenko said on February 20 that more than 20 Russian tanks, 10 missile systems, and busses full of Russian soldiers have crossed into Ukrainian territory during the past 24 hours.
He said the Russian soldiers and heavy weapons were seen moving toward Novoazovsk, just to the east of the strategic governmnet-controlled town of Mariupol on the Sea if Azov.
Hollande said he could not confirm Lysenko's claims of fresh Russian military deployments into Ukrainian territory.
Moscow continues to deny that it is arming eastern Ukraine's pro-Russian separatists and sending Russian soldiers into Ukraine, despite growing evidence that it is doing so.
Ukraine's military also said on February 20 that two of its soldiers were killed in fighting during the previous 24 hours.
It said Ukrainian positions were shelled overnight, including troops in the village of Kurakhove to the west of the rebel stronghold of Donetsk, and in Berdyansk, near Mariupol.
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Separatists reported artillery attacks by Ukrainian forces during the past day, including barrages targetting their positions at Donetsk's airport.
Rebels said they took control of a number of localities around the strategic town of Debaltseve, which government troops evacuated on February 18 following weeks of intense fighting.
Eduard Basurin, an official of the defense ministry of the self-proclaimed Donetsk People's Republic, later said “the situation is stabilizing along the entire line of contact.”
“All is quiet there,” he added.
Under the Minsk deal, meant to quell a conflict that has killed more than 5,600 people since April 2014, both sides were due to lay down their arms on February 15 and begin withdrawing heavy armor from the front lines two days later.
The cease-fire has been broadly observed in eastern Ukraine, while there has been no confirmation of either side pulling back heavy weapons from the front lines.
The West widely condemned the rebel advance in Debaltseve, saying it violated the cease-fire.
But Russia and the separatists say the offensive didn’t defy the truce because the town was encircled by rebel forces at the time of the agreement, something Kyiv rejects.
NATO's top military commander, U.S. Air Force General Philip Breedlove, said he did not think the truce had ever even begun. "It is a cease-fire in name only," Breedlove said during a visit to Kosovo.
Breedlove also said that Russia has helped creating "a formidable army" in eastern Ukraine.