US President Barack Obama has announced a $1bn effort to boost US military presence across Europe to ease anxiety among NATO allies wary of Russia.
He has also said more sanctions are being prepared against Russia if the West continues to see it destabilising Ukraine.
Obama announced the initiative on Tuesday during a visit to Polish capital of Warsaw, his first stop on a three-country tour through Europe, calling for the US Congress' backing on the issue.
Tension remains high in eastern Ukraine, a day after hundreds of pro-Russian separatists attacked a border guard base in Luhansk city following reports of air raids by Ukraine's army on rebel positions.
Speaking in an aircraft hangar at Warsaw airport where he met US airmen taking part in a joint programme with the Polish air force, Obama said US commitments to Poland and the region were a cornerstone of the US’ own security.
"As friends and allies we stand united together," he said.
Obama also said that more sanctions were being prepared against Russia if West continued to see it destabilising Ukraine.
Funding for military drills
The White House said in a statement that the funding would be used to increase military exercises and training missions, as well as rotations of air and ground forces, on the continent.
The statement said Obama was also seeking to reinforce US navy participation in NATO deployments in the Black and Baltic Seas, plus working to boost the military capacity of non-NATO countries that sit on Russia's border, including Ukraine, Georgia and Moldova, Reuters news agency reported.
Obama would be seeking the support of the US Congress for the plan, it said.
The military assistance proposed by the White House, called the European Reassurance Initiative, is to include greater US participation in training and exercises, deploying US military planners, and more persistent naval deployments in the Black Sea and Baltic Sea, on Russia's doorstep.
The US statements came a day after hundreds of pro-Russian separatists fired rocket-propelled grenades at a border guard base in Luhansk, prompting a reported deployment of air support by government forces.
At least five of the attackers were killed in fighting around the base, a spokesman for the guards told the AP news agency.
Al Jazeera's David Chater, reporting from Luhansk on Tuesday, said the general opinion was that Monday's offensive on the border base was a diversionary attack to allow more separatists to cross the border from Russia.
"Border guards are still stuck in the building, but the fighting is not going on. However, it can pick up anytime," he said.
In a separate incident on Monday, a blast at an administrative building in Luhansk held by separatists claimed several lives.
A health official for the Luhansk region told Russia's Interfax news agency that at least seven people had been confirmed dead in the blast.
Rebel forces claimed that Ukrainian jets had responded to the attack on the border post by dropping cluster bombs on the administrative buildings occupied by pseudo-administration in Luhansk while Ukrainian authorities denied carrying out a strike and said the blast was caused by misdirected rebel fire from a portable surface-to-air missile launcher.
A video showed an explosion, debris scattered around the building and men trying to move a body.
Al Jazeera's Chater said at least 10 craters were created by the army's air strike.
"There is real anger happening here against the government," he said.
"Government claims that attack happened through missiles that came from rebels, which is clearly not. It is understandable from the scene."