NATO Secretary-General Jens Stoltenberg has dismissed as "nonsense" accusations made by the Russian president that the Ukrainian Army is acting as the Western alliance's "foreign legion."
Stoltenberg told reporters in Brussels on January 26 that the foreign forces in Ukraine are Russian.
He spoke after Vladimir Putin told university students in St. Petersburg, "Essentially it is not the army at all, but a foreign legion -- in this case, it is a foreign NATO legion,” whose main purpose is to "contain" Russia.
Putin indicated he was referring to volunteer battalions fighting within the Ukrainian Army.
Putin claimed that "many" Ukrainian men were trying to "dodge the mobilization" and sought to leave for Russia.
Putin said the period of stay in Russia could be increased for Ukrainian citizens, primarily those of conscription age.
Meanwhile, Western governments stepped up pressure on Moscow to rein in Russian-backed separatists as the casualty toll mounted on January 26 after the rebels announced a new offensive in eastern Ukraine.
The Kremlin responded angrily to criticism from the United States and EU nations, blaming Kyiv for a flare-up that has killed some 300 people in two weeks and warning the West that attempts to "blackmail" Russia by threatening tighter economic sanctions would fail.
The sharp words underscored the huge challenges facing diplomatic efforts to end the conflict, which has claimed more than 5,000 lives since last April and touched off the biggest crisis in East-West relations since the Cold War.
German Foreign Minister Frank-Walter Steinmeier said Russia must give a clear order to separatists in eastern Ukraine to comply with a cease-fire deal signed in September in Minsk, saying time is running out for the implementation of the peace plan.
The warning came as fighting persisted after a senior leader of the Russian-backed separatists announced plans to seize more ground from government forces on January 23 and rocket attacks blamed on the rebels killed 30 people in Mariupol the following day.
Ahead of a meeting of EU foreign ministers on January 29 to address the conflict, Steinmeier said that nobody was blindly set on imposing further sanctions on Moscow over its interference in eastern Ukraine, where NATO says Russia has sent troops and weapons to help the rebels.
But Steinmeier said that a rebel offensive on Mariupol -- which lies between separatist-held land in eastern Ukraine and Crimea, the peninsula seized by Russia in March -- constitute a significant change in the situation and the West would have to react.
U.S. President Barack Obama said on January 25 that the United States is considering all options short of military action to turn up the pressure on Russia, which has been hit with several rounds of EU and U.S. sanctions since its illegal annexation of Crimea.
Obama spoke a day after rocket attacks blamed on the rebels killed 30 people and wounded dozens more in Mariupol. He also said that Washington would be "in close consultation with our international partners, particularly European partners" on how to proceed with regard to Ukraine.
In Warsaw, Polish President Bronislaw Komorowski said, "The response of the Western world should be very firm."
"In my opinion the EU response should be to deliberately raise the issue of toughening sanctions against Russia," he told a news conference on January 26.
A spokesman for EU foreign policy chief Federica Mogherini said that, at the January 29 meeting, EU foreign ministers "will assess the developments of the ground and see how to respond to this.
Spokeswoman, Maja Kocijancic, said sanctions against Russia could be "scaled up" or "scaled down" if "the situation on the ground so warrants."
Russian President Vladimir Putin's spokesman, Dmitry Peskov, said that threats to increase economic pressure on Russia were "absolutely destructive, completely unjustified, and in the end shortsighted."
He said that "such blackmail never has and never will cause Russia...to change its consistent and well-known policy," the state-run RIA news agency reported.
Russia has blamed Kyiv for the escalation in fighting. Foreign Minister Sergei Lavrov, speaking at a news conference after talks with Israeli Foreign Minister Avigdor Lieberman, claimed that the rebels were responding to attacks by government forces.
Western governments rejected that assertion.
Obama said that "separatists with Russian backing, Russian equipment, Russian financing, Russian training and Russian troops" were to blame.
Fighting continued on January 26, with the Ukrainian military saying at least seven of its soldiers had been killed and 24 wounded in the previous 24 hours.
Military spokesman Vladyslav Seleznyov said rebels had fired on government positions or other targets more than 100 times.
Seleznyov said fighting was at its most intense around the town of Debaltseve, a key road junction between Donetsk and Luhansk that the rebels have vowed to encircle.
Ukraine's ambassador to NATO was to meet with envoys from the NATO member nations in Brussels on January 26 to discuss the situation.
The separatist offensive came despite an agreement reached in Berlin by the foreign ministers of Russia, Ukraine, Germany, and France on January 21 for government troops and the rebels to pull heavy weapons back from a separation line established under the Minsk deal.