NATO said on Thursday that Russia is yet again gathering its troops near the Ukrainian border, a claim that Russia denied, even as seven Ukrainian troops and 300 pro-Moscow separatists were reported killed in clashes in the country's restive east.

NATO’s Secretary General Anders Fogh Rasmussen said that Russia has sent a few thousand troops to the Ukrainian border, as Kiev launched a major strike against pro-Moscow rebels fighting to secede from Ukraine. The fighting continued on Friday in the eastern city of Donetsk despite expectations that clashes would subside after Ukraine’s President Petro Poroshenko announced a cease fire on Wednesday, and reports emerged of a phone call between the Ukrainian leader and Russian President Vladimir Putin.

Rasmussen called Russia’s move “a very regrettable step backward," according to Associated Press, or AP. “If they're deployed to seal the border and stop the flow of weapons and fighters that would be a positive step. But that's not what we're seeing.”

Russia, which faces the threat of further sanctions from the West for its involvement in the region's crisis, reportedly dismissed NATO’s claims saying that its troops, stationed for routine training, were nowhere near the Ukrainian border.

On Thursday, Putin also spoke to German Chancellor Angela Merkel and French President Francois Hollande, and reportedly expressed concern about Ukraine’s offensive against pro-Russian rebels, following Russia’s resolution in the U.N. Security Council requesting the international organization’s help in stemming the violence in eastern Ukraine. Merkel and Hollande reportedly told Putin to implement “every necessary effort” to convince the separatists to stop fighting.

“Otherwise, new measures will be adopted by the international community that will impact its relationship with Russia," both leaders reportedly told Putin, according to the Wall Street Journal.

Vladislav Seleznev, the spokesperson for Ukrainian troops in the eastern part of the country, reportedly said Friday that nearly 300 pro-Russian rebels were killed in clashes on Thursday, according to AP.

Meanwhile, Western leaders have attempted to broker a dialog between the two countries and are trying to convince Russia to accept Poroshenko’s plans for a temporary, unilateral cease-fire until Kiev can regain control of its eastern border.

"We are in a moment where Russia has a fundamental choice to make," Jack Lew, the U.S. Treasury Secretary, said according to the Journal, adding: "We certainly hope that Russia will take the discussions between President Poroshenko and President Putin seriously and move to a diplomatic resolution."

According to estimates by the U.N., at least 356 people have been killed in eastern Ukraine since May 7, of which 257 people were civilians.