US President Barack Obama has said he is still considering whether to arm Ukraine in its conflict against Russian-backed rebels but said he still hoped for a diplomatic solution.

"The measure by which I make these decisions is: 'Is it more likely to be effective than not?'," Mr Obama said at a news conference with German Chancellor Angela Merkel, who opposes providing Ukraine with weapons to fight separatists in its east.

Russia had violated a September peace deal by sending more tanks and artillery into eastern Ukraine, Mr Obama said, adding that he and Mrs Merkel had agreed sanctions must stay for now and Moscow's isolation would worsen if it continued on its current course.

Mrs Merkel is due to meet Russian President Vladimir Putin and Ukrainian President Petro Poroshenko on Wednesday with French President Francois Hollande, and EU ministers have held off tightening sanctions to give the talks a chance.

Mrs Merkel and Mr Obama have come under fire from US foreign policy hawks in the Republican-controlled Congress who want weapons sent to the Ukraine army.

The US Senate and House unanimously passed legislation in December that authorized sending arms for Kiev. Mr Obama signed the measure into law but it gave him leeway over whether or when to implement it.

"The Ukrainians are being slaughtered and we're sending them blankets and meals," Senator John McCain said at a security conference in Munich at the weekend. "Blankets don't do well against Russian tanks."

Nine Ukrainian troops have been killed in a 24-hour period over Sunday and seven civilians also died, with fighting particularly intense around the town of Debaltseve, a major rail and road junction northeast of the city of Donetsk.

Last week Mrs Merkel and Mr Hollande met Mr Putin in Moscow and followed this up with a conference call on Sunday also including Mr Poroshenko. But so far no breakthrough has emerged in the nearly year-long conflict that has claimed more than 5,000 lives.

EU foreign ministers have approved visa bans and asset freezes on more Ukrainian separatists and Russians. But they will wait at least until February 16 before imposing the measures to give peace efforts more time, French Foreign Minister Laurent Fabius said.

Sanctions imposed in stages by the EU and US since Moscow annexed Crimea from Ukraine last March have badly hurt the Russian economy, adding to problems created by the plunging price of oil, the country's main export.

Moscow has warned Mr Putin will not be spoken to in the language of ultimatums. Asked about media speculation Mrs Merkel had issued him an ultimatum at talks on arranging a summit on Ukraine, Kremlin spokesman Dmitry Peskov said: "Nobody has ever talked to the president in the tone of an ultimatum - and could not do so even if they wanted to."

Mr Obama has to decide whether to supply weapons, impose tougher sanctions on Russia in the hope of forcing Mr Putin to compromise, or throw his full weight behind the revised German-French peace initiative.

Some of his top advisers, including Ashton Carter, his choice for new defence secretary, increasingly favour providing items like anti-tank weapons, small arms and ammunition.

National security adviser Susan Rice said arms supplies were under consideration but signalled caution, and stressed the need to maintain unity with European allies.

Such a step would be taken only "in close consultation and in coordination with our partners, whose unity on this issue with us thus far has been a core element of our strength in responding to Russia's aggression", she said.