Washington - Democratic White House hopeful Barack Obama said on Thursday the U.S. friendship with Israel was "unbreakable" and vowed to ensure the security of the Jewish state if elected president.

"America's commitment to Israel's security is unshakable," Obama said to cheers and applause during a brief speech at a celebration to mark the 60th anniversary of the founding of Israel. "I am absolutely convinced that our friendship between the two nations is unbreakable."

Obama, who leads Hillary Clinton in the drawn-out fight for the Democratic nomination, gave few details of his plans for the Middle East but vowed to work to ensure a secure Israel.

"I pledge to you that I will do whatever I can in whatever capacity to not only ensure Israel's security, but also to ensure that the people of Israel may thrive and prosper and build on the enormous promise that was made 60 years ago."

Critics have raised doubts about Obama's commitment to the Jewish state, floating rumors that he was a Muslim and linking him to Louis Farrakhan, a U.S. political figure known for his anti-Israel rhetoric.

Obama is a Christian and has denounced Farrakhan. He has vowed not to change staunch American support of Israel -- the mainstay of U.S. Middle East policy.

Obama's advisers have criticized U.S. President George W. Bush for taking a low-profile approach during his first seven years in office and for not pushing hard to follow up on the Annapolis peace summit in November.

In the Arab world, where many view U.S. policy as biased toward Israel, there is intense interest in whether Obama's approach to the Middle East would be different.

Some foreign policy conservatives in the United States have questioned Obama's approach on the Middle East, criticizing his call for direct talks with U.S. foes like Iran and suggesting he would be more inclined than other presidential candidates to pressure Israel to make concessions toward the Palestinians.

Obama made no mention of any such position.

"As we celebrate six decades of independence, we know that more work remains to be done to secure a lasting peace for the children of Israel," said Obama, who noted that he had visited Israel in 2006 and was struck by the resolve of people who were faced with constant danger and uncertainty.

"So let us honor the independence of this great nation; let us celebrate the achievements of six decades; and let us renew the friendship between our nations, and the solemn promise to seek lasting peace and security for the people of Israel."

Obama appeared shortly before U.S. Vice President Dick Cheney. Obama often jokes about revelations that he and Cheney are distantly related -- getting a laugh when he says he wished genetic tests would have discovered he was related to someone else.

(Additional reporting by Paul Eckert; Editing by Eric Beech)