Ukraine and the pro-Russian separatists appear increasingly close to signing a deal to end four months of fighting.
Ukrainian president Petro Poroshenko said he is ready to order a ceasefire in the east tomorrow if a peace deal is signed that day at talks in Minsk, Belarus.
The rebels also said they were ready to declare a truce if an agreement with Ukraine is reached on a political settlement for the mostly Russian-speaking region.
Mr Poroshenko discussed the outlines of a deal with Russian president Vladimir Putin yesterday, and they both voiced optimism about reaching an agreement in Minsk.
A White House official said Mr Obama and other leaders attending a Nato summit in Wales expressed solidarity with Ukraine and agreed Russia should be punished for its conduct in Ukraine.
"The leaders reiterated their condemnation of Russia's continued flagrant violation of Ukraine's sovereignty and territorial integrity, and agreed on the need for Russia to face increased costs for its actions," US deputy national security advisor Ben Rhodes said. "The leaders also expressed their strong support for President Poroshenko's efforts to achieve a peaceful resolution to the conflict."
Mr Poroshenko was meeting later with the heads of state and government from all 28 Nato member states, even though Nato officials have made clear that membership for Ukraine is not in the cards any time soon.
In Moscow, Russian foreign minister Sergey Lavrov warned that reports Ukraine was seeking to join Nato were "a blatant attempt to derail all the efforts" to seek a peaceful solution to the fighting.
Russian-backed separatists have been battling government troops in eastern Ukraine since mid-April in a conflict that the UN estimates has killed nearly 2,600 people. Nato says at least 1,000 Russian fighters are helping the rebels and Associated Press journalists have seen convoys of heavy military equipment moving in rebel-held territory from the direction of Russia.
Rebels have made substantial advances against Ukrainian forces over the past two weeks, including opening a new front along the Sea of Azov coast. That offensive has raised concerns the rebels are aiming to seize Mariupol, a major port of about 500,000 people, and create a land corridor between Russia and Crimea, the Black Sea peninsula that Russia annexed in March.
An AP reporter saw three military-type vehicles ablaze in Berezove, a village along the main road connecting Mariupol with Donetsk, the largest rebel-held city. Rebel fighters were on the move, indicating they could be trying to take control of the strategic highway. Later, columns of smoke rose outside the nearby village of Olenivka, suggesting that Ukrainian forces were trying to retake it.
Ukraine's UN ambassador said a convoy of Russian tanks and armoured vehicles had moved from the eastern resort town of Novoazovsk toward Mariupol and was stopped by Ukrainian forces and civilians. Yuriy Sergeyev told reporters in New York that "the fight is still going on" and said the convoy had four tanks, three armoured vehicles and about 50 soldiers.
Specifics of the hoped-for peace deal are yet to be finalised. Mr Putin has suggested that rebels halt their offensive while the Ukrainian government forces should pull back away from shelling residential areas.
Mr Poroshenko, in his turn, called for the withdrawal of foreign troops, a diplomatic reference to Russian forces, as well as establishing a buffer zone on the border and releasing all Ukrainian prisoners held in Russia.
Both sides have expressed readiness for international monitoring of the truce and a prisoners' exchange.