WASHINGTON — President Barack Obama is reconsidering his opposition to giving Ukraine defensive weapons and other lethal aid to help its struggling military repel Russian-backed rebels, a possible escalation that has had strong support from many in his national security team.

The shift suggests the White House is growing increasingly concerned that its reliance on punishing Russia with economic sanctions isn't doing enough to change President Vladimir Putin's thinking about backing fighters in ethnic-Russian eastern Ukraine.

A senior Obama administration official said the president still sees pitfalls in plans to send defensive lethal aid to Ukraine, and a decision on the matter is not imminent. However, the official said a recent spike in violence between Ukraine and Russian-backed separatists has sparked a fresh examination of U.S. policy.

The president's worries about sending higher-powered equipment to Ukraine are threefold, according to the official. He sees risk in starting a proxy war between the U.S. and Russia, which the West accuses of supplying rebels in eastern Ukraine. He is worried that the Ukrainian military may not be well-trained enough to effectively use U.S. equipment and believes no amount of arms would put Ukraine on par with the Russian military.

Obama has weighed sending lethal aid to Ukraine before, but has always decided against taking that step. But holding fast to that position has left him isolated within his administration, given the support for sending the Ukrainians defensive assistance from high-ranking officials including Secretary of State John Kerry and NATO Commander Gen. Philip Breedlove.

On Monday, several former U.S. diplomatic and military officials released a report calling on the White House and Congress to give Ukraine $3 billion in military assistance over the next three years. Among the officials who wrote the report are former U.S. Ambassador to Ukraine Steven Pifer and former undersecretary of defense Michele Flournoy.

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

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

Kerry plans to be in Kiev on Thursday to meet with Ukrainian leaders, though administration officials downplayed the notion that his trip would coincide with new announcements on U.S. policy.

Obama has sought to coordinate the U.S. response to the Ukraine crisis with Europe, which he has long considered to have a closer stake in the fight. An official said the president indeed wants to discuss the prospect of lethal aid with his European counterparts, including German Chancellor Angela Merkel, who is due to visit Washington next week.

But 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.

Obama and European leaders have largely centered their efforts to stop Russia's year-long advance on Ukraine on sanctions targeting Moscow's defense, energy and financial sectors, as well as individuals close to Putin. The sanctions, along with the plummeting price of oil, have damaged Russia's economy, but done little to stop the fighting.

A cease-fire deal signed in September has been repeatedly violated by both the Ukrainian government and the rebels. Violence has escalated since the beginning of the year, with the separatists making notable strides in clawing territory away from the government in Kiev.

U.S. officials were particularly troubled by last weekend's attacks on the city of Mariupol, where rockets crashed into a densely populated area, killing 30 people and wounded several dozen others.

Russia has acknowledged that some of its citizens are fighting among the rebels as volunteers, but rejects the Ukrainian and Western charge that it's backing the insurgency with troops and weapons.

The U.S. has so far limited its military assistance to Ukraine to non-lethal equipment, including gas masks and radar technology to detect incoming fire. 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 ratcheting up U.S. and European sanctions.

All of the officials insisted on anonymity because they were not authorized to publicly discuss the U.S. policy deliberations.

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."

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Associated Press writers Matthew Lee, Lolita C. Baldor and Robert Burns in Washington and John-Thor Dahlburg in Brussels contributed to this report.

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