President Barack Obama says the US is studying the option of supplying lethal defensive arms to Ukraine if diplomacy fails to end the crisis in the east.
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.
Ukrainian government officials say nine soldiers and at least seven civilians have been killed in fighting over the last 24 hours.
The crisis in Ukraine has already claimed more than 5,300 lives and displaced 1.5 million people from their homes.
Mrs Merkel’s US trip is the latest in a flurry of diplomatic activity to try end nearly a year of conflict in Ukraine
Mrs Merkel met the US president in Washington on Monday to update him on Franco-German efforts to revive last year’s Minsk peace plan, which collapsed amid fighting over the winter.
The detailed proposals have not been released but the plan is thought to include a demilitarised zone of 50-70km (31-44 miles) around the current front line.
Four-way talks between Russia, Ukraine, Germany and France are due to be held in Belarus’s capital Minsk on Wednesday to discuss the proposals. BBC