Ukrainian President Petro Poroshenko has accused Russia of sending more than 9,000 troops into Ukrainian territory and has demanded that they leave.
Speaking at the World Economic Forum in Davos, Switzerland, on January 21, Poroshenko demanded that Russia immediately implement all of its obligations under the Minsk peace plan, close Russia's border with Ukraine, "and withdraw all the foreign troops from my territory."
Poroshenko said that in addition to the thousands of troops in Ukraine, Russia had about "500 tanks, heavy artillery, and armored personnel carriers."
"If this is not aggression, what is aggression?" Poroshenko asked.
Earlier, Poroshenko said in an interview that Russia had sent additional Russian troops, tanks, and armored personnel carriers into Ukrainian territory in recent days.
He said the Russian military deployments were clearly a form of aggression.
NATO Secretary-General Jens Stoltenberg said on January 21 in Brussels that the alliance had noted a recent increase in the number of tanks, artillery, and other heavy military equipment being used by Russian troops in eastern Ukraine.
Stoltenberg, who said NATO had monitored Russian troop movements in eastern Ukraine for several months, declined to give specific numbers of troops.
Russian Foreign Minister Sergei Lavrov dismissed those allegations as baseless.
In Washington, U.S. Secretary of State John Kerry accused the separatists of attempting "a blatant land grab" in eastern Ukraine.
Kerry, speaking on January 21 after talks with EU foreign-policy chief Federica Mogherini, said Washington was "particularly concerned" by a rebel initiative to "attempt to gain control of a very significant rail juncture" in eastern Ukraine in violation of the Minsk agreement.
Kerry said the move was "an effort to try to broaden the amount of territory that is being held."
The latest statements came ahead of a meeting in Berlin of the foreign ministers of Russia, Ukraine, France, and Germany to discuss the conflict in eastern Ukraine.
Speaking in Berlin, German Chancellor Angela Merkel said on January 21 that she hoped the talks would lead to improving the cease-fire in eastern Ukraine but that "I don't want to get hopes up too much."
She added that the cease-fire was becoming "more and more fragile."
Ukraine said on January 20 that its troops had been attacked by regular Russian Army troops in the northern part of the Luhansk region.
An AP correspondent in the separatist-held part of Luhansk reported seeing six antitank cannons, 15 tanks, nine self-propelled howitzers, and Grad multiple-rocket launchers heading toward Checkpoint 31, where Ukrainian and rebel fighters had fought on January 20.
Ukraine announced plans on January 21 to increase its military strength by 68,000 troops.
Poroshenko also demanded the immediate release of all Ukrainian prisoners captured during the conflict with pro-Russian rebels in eastern Ukraine, including pilot Nadia Savchenko, who has been on a hunger strike in a Russian jail since December 15.
He said the prisoners were "hostages who are illegally held in the occupied territories and in Russian prisons."
Earlier on January 21, Parliamentary Assembly of the Council of Europe (PACE) President Anne Brasseur also asked the Kremlin-allied speaker of Russia's lower house of parliament to help secure Savchenko's release.
Meanwhile, Poroshenko added from Davos that Ukraine will not renegotiate the cease-fire agreement signed in Minsk in September.
The Ukrainian president also said on January 21 that his country will break its dependence on Russian natural gas by 2017.
He said that would be possible by receiving more gas from European sources and by using shale-gas technology within Ukraine.
With reporting by AFP, Reuters, and AP