WASHINGTON -- President Barack Obama and German Chancellor Angela Merkel declared Monday that Russian aggression in Ukraine has only reinforced the unity of the U.S. and Europe, as they weighed the prospects of reviving an elusive peace plan to end the conflict.

Still, Obama held open the prospect that if a new round of diplomacy this week fails, the U.S. could send Ukraine's beleaguered military defensive weaponry. The president said that while he has yet to make a decision on lethal aid, his team is considering "whether there are additional things we can do to help Ukraine bolster its defences in the face of Russian aggression."

Merkel and other European leaders staunchly oppose arming Ukraine, in part out of fear of sparking a proxy war with Russia.

The U.S. and Europe have largely been in agreement on their response to the conflict between Russia and Ukraine, raising the prospect that a public split over lethal aid is a tactic to push Russian Vladimir Putin to agree to a peace plan.

During a joint White House news conference with Obama, Merkel reaffirmed that she sees no military solution to the fighting in eastern Ukraine. However, she added that no matter what Obama decides, "the alliance between the United states and Europe will continue to stand, will continue to be solid."

Merkel and French President Francois Hollande met with Putin and Ukrainian leaders last week and announced a new summit meeting for Wednesday in Minsk. The United States was not at the negotiating table last week, nor will it participate in Wednesday's talks.

Merkel, who has perhaps the most productive relationship with Putin of any Western leader, said reaching a diplomatic agreement was crucial to keeping the peace in Europe.

"I myself actually would not be able to live without having made this attempt," she said through a translator.

Merkel was to head to Ottawa on Monday as well, to give Prime Minister Stephen Harper an update on her transatlantic shuttle diplomacy on the Ukraine crisis.

More than 5,300 people have been killed since fighting in eastern Ukraine began in April, according to a U.N. tally. The bloodshed has markedly increased over the past two weeks, sparking both the new diplomatic manoeuvring and Obama's re-evaluation of sending Ukraine defensive military aid.

The president gave no indication of how quickly he would make a decision on possibly ramping up military assistance to Ukraine, nor did he indicate whether there was a specific development that might trigger that step.

"The measure by which I make these decisions is, is it more likely to be effective than not," he said.

The U.S. has so far limited its military assistance to non-lethal equipment, including gas masks and radar technology to detect incoming fire. If Obama approves lethal aid, the U.S. could send Ukraine anti-tank missiles, such as the Javelin weapon system, along with armoured vehicles.

The U.S. and Europe have largely focused their punitive measures against Russia on several rounds of economic sanctions. The penalties, along with plummeting oil prices, have caused significant damage to Russia's economy.

The European Union decided Monday to temporarily hold off on ordering more sanctions on Russians and Ukrainian separatists while awaiting the outcome of the peace talks.

Details of the proposals being discussed between Russia, Ukraine, Germany and France have not been revealed, but the main sticking points have emerged in the leaders' recent comments.

One is enforcing a peace deal. Ukrainian President Petro Poroshenko has expressed opposition to any peacekeeper force, apparently reflecting concern that sending Russian peacekeeping troops into eastern Ukraine could result in a de facto occupation.

However, key to a real settlement is some mechanism for monitoring the Ukraine-Russia border to ensure that Russia is not sending troops or equipment to the separatists. Ukrainian officials would have the Organization for Security and Co-operation in Europe conduct such monitoring.

The status of the eastern regions remains contentious. Ukraine passed a law last year proposing what it called significant autonomy for the east, but rebels dismissed it as vague and meaningless. Russia has pushed for "federalization" of Ukraine, which would presumably give the east significant independence, but Ukrainian authorities oppose that.

Obama and Merkel also discussed the U.S.-led military campaign against the Islamic State militants and the ongoing nuclear negotiations with Iran. The U.S. and Germany, along with their negotiating partners, are discussing with Iran the outlines of a framework agreement ahead of a late March deadline.

The negotiations have been extended before. Obama said Monday that he does not believe an extension would be "useful" unless Iran can agree to the basic outlines of a deal.

"They should be able to get to yes," Obama said. "But we don't know if that's going to happen."