The NATO military alliance, at its summit in Wales, has said it “stands with Ukraine” in the face of Russia’s “destabilising” influence.
Nato also called on Russia “to pull back its troops” from Ukraine and end the “illegal” annexation of Crimea.
Russia denies arming pro-Russia rebels in eastern Ukraine or that it has deployed troops in the country.
Separately, Ukrainian President Petro Poroshenko and rebel leaders said a ceasefire could be agreed on Friday.
Some 2,600 people have died in fighting between Ukrainian troops and rebels. Ukraine’s military says 837 its soldiers have died since the conflict erupted in April.
In a statement delivered by Nato Secretary-General Anders Fogh Rasmussen, the alliance also called on Russia “to step back from confrontation and take the path to peace”.
Nato’s partnership with Ukraine was “strong” and the alliance was “determined to make it even stronger”, including developing the ability of Ukrainian and Nato forces to work together.
Ukrainian government forces have recently suffered several losses of territory, after rebels launched offensives in both the Luhansk and Donetsk regions, and further south around the city of Mariupol on the Azov Sea.
Reports are emerging that the separatists have begun shelling the outer defences of Mariupol. At least two military vehicles were seen on fire in the area, and eyewitnesses spoke of gunfire.
Meanwhile, President Poroshenko said “the implementation” of a peace plan – which includes a bilateral ceasefire – could start on Friday, a hope expressed earlier by Russia’s President Putin.
The Ukrainian leader said this depended on planned talks in Minsk between representatives of Ukraine, Russia, the pro-Russian rebels and the Organization for Security and Co-operation in Europe.
Rebel leaders were later quoted as saying they would order a ceasefire at 11:00 GMT on Friday if the peace plan was agreed.
On Wednesday, Mr Putin announced a seven-point plan, including a halt to “active offensive operations” by the Ukrainian military and pro-Russia rebels, international ceasefire monitoring, unconditional prisoner exchanges and humanitarian aid corridors.
During the two-days of talks in Wales, Nato leaders are also set to discuss the rise of Islamic State (IS), and Afghanistan where Taliban militants launched a deadly attack on a government compound on Thursday.
Writing in the Times newspaper, UK Prime Minister David Cameron and US President Barack Obama said they would “confront” IS, warning their countries would “not be cowed by barbaric killers”.
Their statement follows the release of a new IS video showing the killing of US journalist Steven Sotloff, just days after the group beheaded another American reporter, James Foley.
In the latest video, an IS militant is also seen threatening to kill a UK hostage, aid worker David Haines, who was seized in March 2013 in Syria’s Idlib province.
Earlier on Thursday, Rasmussen said the summit was taking place in a “dramatically changed security environment”, with Russia “attacking Ukraine”.
“We are still witnessing unfortunately Russian involvement in destabilising the situation in eastern Ukraine,” he told journalists in Newport ahead of the summit’s official launch.
Cameron stressed pressure of sanctions was “the right way to tell the Russians that what they are doing is unacceptable”.
The summit is considered Nato’s most important for decades, as leaders faced the question of whether the alliance is equipped to deal with 21st Century challenges.
Nato is expected to approve plans to create a rapid response force composed of several thousand troops from member states, able to deploy within 48 hours.
Moscow’s actions in Ukraine have raised fears among Nato states bordering Russia, including Lithuania, which has a sizeable Russian-speaking minority.
Lithuanian President Dalia Grybauskaite told the BBC that if Nato did not face up to the challenge “sooner or later we will be facing the aggressor near our border, literally”.
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