Sergei Chuzavkov, Associated Press
Ukrainian soldiers carry the coffin bearing the body of serviceman Ruslan Baburov, who was killed in fighting against Russian-backed separatists, during a commemoration ceremony in Independence Square in Kiev, Ukraine, on Monday, Feb. 2, 2015. Ukraine's government said Sunday that 13 of its troops were killed and another 20 wounded in a day of fighting across the east.

WASHINGTON — President Barack Obama is reconsidering whether to send lethal assistance to Ukraine, a senior administration official said Monday, but continues to have concerns about the effectiveness of that step and the risks of a proxy war between the U.S. and Russia.

The official said Obama is specifically concerned about the besieged Ukrainian military's capacity for using high-powered, American-supplied weaponry. The president has also argued that no amount of arming the Ukrainians would put them on par with Russia's military prowess.

The U.S. accuses Russia of supplying the pro-Kremlin separatists that are stirring instability in eastern Ukraine. The U.S. has limited its supplies to the Ukrainian military to non-lethal aid, such as gas masks and radar technology to detect incoming fire.

Some administration officials, including Secretary of State John Kerry and NATO Commander Gen. Philip Breedlove, have been pressing Obama for months to expand that assistance to include defensive aid to help Ukraine's military hold positions and prevent more incursions by Russian-backed separatists.

While the White House has weighed this option previously, Obama has stuck with his opposition to lethal aid. However, the official said an uptick in violence in eastern Ukraine in recent weeks has spurred Obama to take a fresh look at supplying Ukraine with lethal aid, along with other options for calming tensions.

A U.S. military official said defensive lethal aid could include anti-tank missiles, such as the Javelin weapon system, along with armored vehicles. Other options could involve foreign military sales, training or other aid. The U.S. and Europe could also ratchet up economic sanctions against Russia.

The officials insisted on anonymity because they were not authorized to discuss the matter publicly.

A decision on escalating aid is not imminent, the administration official cautioned, adding that Obama would want to first discuss the issue with European leaders, including German Chancellor Angela Merkel, who is due to visit Washington next week.

Merkel said Monday that Germany will not provide weapons to Ukraine and prefers economic sanctions and negotiations to "solve or at least mitigate the conflict."

"It is my firm belief that this conflict cannot be solved militarily," Merkel said after meeting with Hungarian Prime Minister Viktor Orban in Budapest.

Secretary of State John Kerry is due to visit Kiev Thursday to meet with Ukrainian officials.

Obama and European leaders have largely centered their efforts to stop Russia's advances on Ukraine on sanctions targeting Moscow's most profitable economic sectors, as well as individuals close to Russian President Vladimir Putin. The sanctions, along with the plummeting price of oil, have hurt Russia's economy, but have done little to change Putin's calculus with regards to Ukraine.

Bernadette Meehan, spokeswoman for the White House National Security Council, said the White House is "constantly assessing our policies in Ukraine."

"Although our focus remains on pursuing a solution through diplomatic means," she said, "we are always evaluating other options that will help create space for a negotiated solution to the crisis."

The top U.S. Army general in Europe, Lt. Gen. Ben Hodges, told reporters in Washington last week after visiting Ukraine that its army needed equipment and training to help it defend against what he called Russian-supplied artillery and rockets. Hodges, based in Germany, did not mention specific defensive weapons but suggested they could include more sophisticated radar systems and related counter-fire equipment, which would enable the Ukrainian army to pinpoint the origin of heavy artillery and rocket fire and quickly attack it.

"That is an area where I think there are material as well as training requirements," Hodges said, adding, "They have suffered a lot of casualties from heavy artillery and rockets."

Since the crisis between Ukraine and Russia began early last year, the U.S.-led NATO alliance has sought to strengthen the Ukrainian military's command, control, communications and computers capabilities, as well as to improve logistics and standardization, cyber defense, military career transition and the rehabilitation of injured troops.

However, NATO officials have also stressed that the alliance has no military hardware of its own. Each of the 28 member nations is free to assist Ukraine as it deems fit.

On Monday, Alexander Vershbow, NATO's top-ranking American civilian official, said that Putin's actions have been a "game-changer in European security," and predicted the challenge from Moscow will be long-term. He said Russia was even less predictable now than during the Cold War.

"Today, we must contend with a Russia that wants to go back to a Europe based on spheres of influence and doctrines of limited sovereignty for its neighbors, policies that are a throwback to an earlier time, a time we thought we had put behind us," Vershbow said in a speech prepared for delivery at the 2015 Leangkollen Conference in Oslo, Norway.

The New York Times first reported the new Obama administration deliberations on lethal assistance.

Associated Press writer John-Thor Dahlburg in Brussels contributed to this report. Follow Julie Pace at