In response to a question posed at the White House, President Obama said the U.S. is exploring "the possibility of lethal defensive weapons" in the conflict between Russia and Ukraine, but that it is unlikely that the conflict will be resolved militarily. (AP)

President Obama said Monday that he is considering providing Ukraine with lethal weapons to defend the country against Russian aggression if diplomatic options fail.

Speaking at a news conference, Obama and German Chancellor Angela Merkel said the United States and Europe remain united in trying to achieve a diplomatic resolution to the crisis and on continuing sanctions against Russia.

A summit involving Germany, France, Russia and Ukraine is scheduled to take place Wednesday, seeking to revive a short-lived peace agreement reached between the Ukrainian government and pro-Russian rebels in September.

Merkel said she and French President Francois Hollande decided to give a peaceful resolution another attempt. She said she saying she would not be able to live with herself if she did not try for one again.

Despite that, Merkel said there is “anything but an ensured success” from the talks, which will be held Wednesday in Minsk, the capital of Belarus.

If success is not possible, she said, the United States and Europe will have to explore “further possibilities.”

Merkel has made it clear that she does not want one of them to involve providing arms to Ukraine. Obama said he is considering a plan to arm Ukraine with defensive weapons but has not yet made a decision and continues to consult with allies.

“What other means can we put in place to change Mr. Putin’s calculus? And the possibility of lethal defensive weapons is one of those options that’s being examined,” Obama said, referring to Russian President Vladi­mir Putin.

“It’s not based on the idea that Ukraine could defeat a Russian army that was determined. It is rather to see whether or not there are additional things we can do to help Ukraine bolster its defenses in the face of separatist aggression,” Obama said.

Both leaders said that no matter what happens, there will continue to be a unified response from the United States and Europe.

“The United States and Europe have not stood idly by,” Obama said, citing an enormous investment of money and diplomacy to try to resolve the situation.

As Obama and Merkel were speaking in Washington, rebels were making new gains on the ground in eastern Ukraine, coming close to completing the encirclement of a Ukrainian-held town that has seen intense fighting for weeks.

German Chancellor Angela Merkel speaks as she holds a joint news conference with President Obama in the East Room of the White House in Washington, D.C., Feb. 9, 2015. (Kevin Lamarque/Reuters)

Rebels on Monday claimed they had seized full control of a key point on the supply line into government-held Debaltseve. Rebel military spokesman Eduard Basurin said 5,000 to 6,000 Ukrainian troops were trapped, and he urged them to surrender, according to Russia’s Interfax news agency.

A Ukrainian military spokesman said the troops were not surrounded and that fighting continued along the vital supply line.

Obama said he and Merkel agreed that “sanctions on Russia need to remain fully enforced until Russia fully complies with its obligations” on the peace treaty. In Brussels on Monday, the European Union delayed imposing a new round of sanctions against Russia ahead of the talks Wednesday.

Obama said that while he believes there are some inside Russia who “recognize that this has been a disastrous course for the Russian economy,” until a cease-fire stands, the United States and Europe are going to have to try different ways to resolve the situation, including every attempt at a diplomatic one. Obama said the world must show Russia that it will incur costs if it continues its aggression.

“We’re going to have to keep on trying different things to see if we can get a better outcome,” Obama said.

Preparations for Wednesday’s talks are taking place in Berlin. A second, parallel convocation is expected to take place at the same time in Minsk and include pro-Russian rebels.

Merkel’s day with Obama, which included an Oval Office meeting, a news conference and a working lunch, had been planned for some time.

Merkel brushed aside a question on alleged U.S. monitoring of her phone in 2013, saying that there are “different assessments on individual issues” but that the two countries need to work together on issues of terrorism and cybersecruity and that the United States provides Germany with “significant, very important information” on security. Obama said the revelations have damaged impressions that Germans may have of the U.S. government and that he has worked to create greater transparency.

When asked about Israeli Prime Minister Benjamin Netanyahu’s plan to visit the United States and address a joint meeting of Congress on March 3, a move that angered the administration, Obama said he has a long-standing practice of not meeting with leaders close to elections. Israeli elections are slated to take place March 17. Obama said Merkel would not have asked for or received an invitation to the White House two weeks before an election.

“Some of this just has to do with how we do business,” Obama said, adding that the way to maintain the “unbreakable bond” between the United States and Israel is to “make sure that it doesn’t get clouded with what could be perceived as partisan politics.”

He reiterated, however, that he and Netanyahu have a “real difference” over whether additional sanctions should be enacted against Iran at the moment. A bill before Congress would expand sanctions. Obama said he would veto it because it would undermine ongoing talks on Iran’s nuclear program and risk setting up a military confrontation.

The agreement to hold Wednesday’s talks came during the Munich Security Conference, an annual event that was dominated this year by Ukraine and divisions over how to stop the escalation of the situation and solve a growing standoff between the West and Russia.

The summit laid bare the differences between how some in the United States and Europe propose responding to the crisis with the issue of arms, but Secretary of State John F. Kerry Sunday played down the idea of a rift between the United States and Europe.

“There is no division; there is no split,” Kerry said. “I keep hearing people trying to create one. We are united. We are working closely together.”

A bipartisan group of senators made it clear this weekend that they believe the only way to break the crisis is to provide Ukraine with weapons.

“We must provide defensive arms to Ukraine,” Sen. John McCain (R-Ariz.) said in Germany.

Merkel and Hollande both traveled to Kiev, Ukraine, and Moscow last week to lay the groundwork for the summit, yet both countries concede that numerous hurdles remain.

“Even after this weekend of intense negotiations, we are far away from a political solution of the Ukraine conflict,” German Foreign Minister Frank-Walter Steinmeier said in Munich this weekend.

Putin said Sunday that the meeting would happen on the condition that the parties agreed to certain points.

“We will be aiming for Wednesday, if by that time we are able to agree on a number of the positions that we recently have been discussing intensely,” he said.

Michael Birnbaum in Moscow contributed to this report.