WASHINGTON (Reuters) - President Barack Obama told Israeli Prime Minister Benjamin Netanyahu he deeply regretted the loss of life in an Israeli raid on a Gaza-bound aid flotilla on Monday and urged him to quickly get to the bottom of the incident.

President Barack Obama speaks in the pouring rain during a Memorial Day event at the Abraham Lincoln National Cemetery in Elwood, Illinois, May 31, 2010. REUTERS/Larry Downing

The White House’s cautious response, which contrasted with an outcry against Israel’s actions in Europe and the Muslim world, reflected a difficult balancing act for Obama.

He faces international pressure to join condemnation of Israel but must also be mindful that the Jewish state, a close U.S. ally, is popular with American lawmakers and voters. At the same time, fledgling U.S.-led Israeli-Palestinian peace efforts are at risk of collapse.

“The president expressed deep regret at the loss of life in today’s incident, and concern for the wounded,” the White House said in a summary of Obama’s phone call with Netanyahu hours after Israeli marines stormed a Turkish aid ship bound for Gaza and at least nine pro-Palestinian activists were killed.

“The president also expressed the importance of learning all the facts and circumstances around this morning’s tragic events as soon as possible,” it said.

Obama, whose relations with Netanyahu have been chilly at times, also told him he understood his decision to cancel their White House talks on Tuesday and return home from a visit to Canada to deal with the crisis. They agreed to reschedule soon, the White House said.

Israel’s storming of the aid ship unleashed international outrage over the bloody end to a bid by human rights campaigners to break an Israeli blockade of the Hamas-controlled Gaza Strip. The U.N. Security Council convened an emergency session on Monday.

Netanyahu said Israeli forces had been attacked while boarding. In addition to the activists killed, seven troops and 20 protesters were injured, the Israeli military said.

TALKS MEANT TO GIVE NUDGE

“At this point, it is unclear what happened and there must be a thorough investigation,” U.S. Senate Foreign Relations Committee Chairman John Kerry said. “This unfortunate incident underscores the necessity of resolving the conflict between Israel and the Palestinians.”

Turkey’s U.S. ambassador, Namik Tan, pressed for a tougher line with Israel. “We would expect our American friends to ... be strongly condemning the Israeli action. So far we haven’t heard that,” he told Reuters.

U.S. Secretary of State Hillary Clinton spoke by phone with Israeli Defense Minister Ehud Barak, who provided “some initial details” of the incident, her spokesman, P.J. Crowley, said.

The postponement of Netanyahu’s U.S. visit saved Obama from the awkward choice of taking his guest to task and risking further fraying of ties or avoiding face-to-face criticism and angering Muslim allies like Turkey.

Obama had hoped to use his talks with Netanyahu to give a nudge to indirect U.S.-sponsored peace talks between Israel and the Palestinians and ease any lingering U.S.-Israeli strains over Jewish settlement construction on occupied land.

Getting long-stalled negotiations back under way marked Obama’s biggest achievement in Middle East diplomacy since taking office last year pledging to make it a high priority.

It is also part of his outreach to the Muslim world, where Obama has sought to counter perceptions of U.S. bias in favor of Israel, especially under his predecessor George W. Bush.

But there has been little or no progress since talks started this month, and with the Gaza flotilla incident, prospects for keeping the process alive look bleak. Obama will have a chance to try when he hosts Palestinian President Mahmoud Abbas, who called the raid a “massacre,” on June 9.

Potential fallout from the incident poses another big headache for Obama, who is already struggling with a massive Gulf of Mexico oil spill and high unemployment at home and nuclear standoffs with North Korea and Iran abroad.