US President Barack Obama has said that his administration is looking at all options in handling the crisis in Ukraine, but he has not yet decided whether the US will provide lethal arms to Kiev.

"The possibility of lethal defence is one of those options that's being examined. But I have not made a decision about that yet," Obama told a joint press conference with German Chancellor Angela Merkel on Monday.

Obama said that his team was considering "whether there are additional things we can do to help Ukraine bolster its defences in the face of Russian aggression".

We're in absolute agreement that the 21st century cannot stand idle, have us stand idle and simply allow the borders of Europe to be redrawn at the barrel of a gun. Barack Obama, US president

During a joint White House news conference the two leaders said that Russian aggression in Ukraine has only reinforced the unity of the US and Europe.

Separately, 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 said that abandoning the principle of territorial integrity at the heart of the Ukraine crisis posed a threat to the "peaceful order of Europe".

"For somebody who comes from Europe, I can only say, if we give up this principle of territorial integrity, we will not be able to maintain the peaceful order of Europe," she said. "It's essential."

Peace talks

Merkel and other European leaders staunchly oppose arming Ukraine, in part out of fear of sparking a proxy war with Russia. The conflict has already left 5,400 people dead in less than a year.

The German leader has sought to negotiate a peace deal with Russian President Vladimir Putin after joining crisis talks in Moscow last week with French President Francois Hollande.

A summit of leaders of Ukraine, Germany, France and Russia is tentatively planned for later this week, after Merkel leaves Washington.

Obama meanwhile, warned that the West could not allow Russia to rewrite Europe's borders "at the barrel of a gun".

"And as diplomatic efforts continue this week, we're in absolute agreement that the 21st century cannot stand idle, have us stand idle and simply allow the borders of Europe to be redrawn at the barrel of a gun."

EU sanctions on hold

On Monday, EU foreign ministers approved visa bans and asset freezes on more Ukrainian separatists and Russians. However, the decision was made to wait at least until February 16 before imposing the measures to give peace efforts more time, according to French Foreign Minister Laurent Fabius.

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

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