NATO is set to create a "high-readiness" force and stockpile military equipment in Eastern Europe as a bulwark against potential Russian aggression, the alliance's chief has said.
Anders Fogh Rasmussen said the proposed new force could be comprised of several thousand troops contributed to on a rotating basis by the 28 NATO countries.
Backed by air and naval assets, he said the unit would be a "spearhead" that could be deployed at very short notice to help NATO members defend themselves against any threat, including from Russia.
NATO leaders are to consider the plans at a summit this week in Wales that is likely to be dominated by how the US-led alliance should respond to the Russian-backed separatist uprising in eastern Ukraine.
The move could provoke Russia, whose foreign minister warned on Monday that Ukrainian forces must pull back from areas where they can harm civilians.
Sergei Lavrov spoke amid reports that Ukrainian forces had been ordered to pull back from Luhansk airport in the face of an onslaught from Russian tanks - the latest claim of direct Russian involvement in the fighting.
Ukraine's Defence Minister Valeriy Geletey said Russian units were moving into other towns in the region, including the largest city of the Donetsk region.
"The information that Russian troops are there has been confirmed," he said.
"We are fighting Russia and it is Russia which is deciding what will happen in Donbass," he told Ukraine's Inter channel, referring to the informal name of the Donetsk and Lugansk regions.
Mr Lavrov again denied that Russian troops were in Ukraine and said he hoped talks taking place on Monday in the Belarussian capital Minsk today will focus on agreeing an immediate, unconditional ceasefire.
Speaking in the east Siberian city of Yakutsk on Monday, Russia's President Vladimir Putin accused Europe of ignoring the Ukrainian army "directly targeting its fire on residential areas".
He said he hoped "common sense will prevail" and that Russia and the West would not harm each other with further sanctions.
While Mr Rasmussen has said NATO will not allow any allies to be attacked.
A rights group that works to expose Russian army abuses has claimed that up to 15,000 soldiers have been sent to Ukraine by Moscow in the last two months, and several hundred may have died in combat.
Valentina Melnikova, head of the Committee of Soldiers' Mothers, a prominent organisation representing the families of military servicemen, said that some 7,000-8,000 Russian troops are believed to be in Ukraine at present.
"Military commanders are conducting a secret special operation," said Melnikova, who is a member of the defence ministry's public council.
Meanwhile, a Ukrainian military spokesman said a rescue operation was continuing for two seamen missing in the Azov Sea after pro-Russian separatists attacked a Ukrainian navy vessel for the first time.
Eight other seamen survived the attack and were being treated for wounds and burns after the vessel was hit by artillery from the shore.
Separatists in the region claimed responsibility for the attack on social media.
Ukraine's President Petro Poroshenko said the events of the past few days have shown that Russia has launched "a direct and open aggression" against Ukraine.
Leading American senators have called for the US to send weapons to help Ukraine defend itself against what they called "a Russian invasion".
Democrat Robert Mendez, who runs the Senate Foreign Relations Committee, told CNN: "We should provide the Ukrainians with the type of defensive weapons that will impose a cost upon Putin for further aggression."
The call was echoed by former Republican presidential candidate John McCain, who told CBS's Face the Nation that Mr Putin was "an old KGB colonel that wants to restore the Russian empire".
Earlier, the European Union gave Russia a week to scale back its intervention in Ukraine, warning of further sanctions. China said it opposes additional sanctions against Russia and has urged world leaders to find a political solution to the crisis.
