Russia had violated "every commitment" made in the failing Minsk agreement, he added, after talks with the German chancellor on a new peace deal.
Mr Obama has come under pressure from senior US officials to supply arms, despite objections from Angela Merkel.
Russia denies accusations of sending troops and supplying the rebels.
The latest diplomatic efforts come amid renewed fighting between the pro-Russia rebels and Ukrainian government troops, with fighting centred around the strategic railway hub of Debaltseve.
The rebels said on Monday they had cut off a key supply road to the town, which is near the rebel-held city of Donetsk, but the military says the battle is ongoing.
Russian President Vladimir Putin, who is considering the peace proposals, earlier blamed the West for the crisis.
The fresh push for talks is aimed at ending a crisis that has already claimed more than 5,300 lives and displaced 1.5 million people from their homes.