US President Barack Obama made clear he was some way from a decision on whether to arm Ukraine in its conflict against Russian-backed rebels, saying on Monday 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?'," 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, Obama said, adding that he and Merkel had agreed sanctions must stay for now and Moscow's isolation would worsen if it continued on its current course.

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

Merkel and 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. Obama signed the measure into law but it gave him leeway over whether or when to implement it.

Congressional aides said on Monday lawmakers were working on legislation that would commit Obama to providing arms.

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

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

But Merkel made clear in Munich her opposition to arming Kiev. "I understand the debate but I believe that more weapons will not lead to the progress Ukraine needs. I really doubt that," she said.

KREMLIN REJECTS ULTIMATUM TALK

A Russian speaker who grew up in East Germany, Merkel has taken the lead in pursuing a diplomatic solution, speaking with Putin by phone dozens of times over the past year and meeting him in Russia, Australia and Italy in recent months.

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

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

"The principle of these sanctions remains but the implementation will depend on results on the ground," he said. "We will see by Monday and see how the meeting in Minsk went."

Sanctions imposed in stages by the EU and United States 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 warned on Monday that Putin will not be spoken to in the language of ultimatums. Asked about media speculation Merkel had issued him an ultimatum at talks on arranging a summit on Ukraine, Kremlin spokesman Dmitry Peskov told Govorit Moskva radio: "Nobody has ever talked to the president in the tone of an ultimatum - and could not do so even if they wanted to."

OBAMA'S OPTIONS

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

He said his decision would not rest on a specific point.

"Can we be certain that any lethal aid that we provide Ukraine is used properly, doesn't fall into the wrong hands, does not lead to over-aggressive actions that can't be sustained by the Ukrainians? What kinds of reactions does it prompt, not simply from the separatists but from the Russians? Those are all issues that have to be considered," he said at the White House.

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

National security adviser Susan Rice said arms supplies were under consideration but signaled 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.