WASHINGTON - US President Barack Obama signed a bill on Friday to strengthen US-Israeli military ties as he sought to reassure American Jewish voters of his commitment to the two countries' close alliance on the eve of a visit to Israel by his Republican rival, Mitt Romney.

Obama used a White House ceremony to announce the United States would soon provide Israel with an additional $70 million in funding for its short-range rocket shield known as "Iron Dome," a project strongly backed by the powerful US pro-Israel lobby.

His focus on strengthening cooperation with Israel appeared timed to upstage Romney, who has accused the president of undermining Washington's relationship with its number one partner in the Middle East.

Romney, whose Olympics-week visit to London has been plagued by diplomatic stumbles, will travel on Saturday to Israel, a stop his aides hope will resonate with Jewish voters at home.

He is expected to hold talks on Sunday with Prime Minister Binyamin Netanyahu, who has had a strained relationship with Obama.

As Obama signed the bill at his desk in the Oval Office, he said it underscored his administration's "unshakeable commitment" to Israel's security. Congress passed the legislation last week with broad support from Republicans and Obama's Democrats.

"I have made it a top priority for my administration to deepen cooperation with Israel across a whole spectrum of security issues," Obama said in the Oval Office.

He was flanked by US Senator Barbara Boxer and Congressman Howard Berman, the bill's sponsors, and several prominent Jewish leaders, including Lee Rosenberg, chairman of AIPAC, the leading pro-Israel lobby, and Richard Stone, chairman of the Conference of Presidents of Major American Jewish Organizations.

Obama, criticized by some of Israel's US supporters for being too tough on a close ally, wants to shore up his support among Jewish voters, who could prove critical in battleground states like Florida and Pennsylvania in the Nov. 6 election.

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