WASHINGTON: US President Barack Obama on Friday said he is releasing an additional USD 70 million in military aid for Israel, a previously announced move that appeared timed to upstage Republican rival Mitt Romney's trip to Israel this weekend.

The stepped-up US aid, first announced in May, will go to help Israel expand production of a short-range rocket defence system. The system, known as Iron Dome, has proved successful at stopping rocket attacks fired at Israeli civilians from close range, including from Gaza.

The official spoke on the condition of anonymity because he was not authorised to speak publicly ahead of the president.

Obama announced the new military assistance as he signed a bill in the Oval Office expanding military and civilian cooperation with Israel.

Obama said the bill underscores the United States' "unshakable commitment to Israel”.

The White House focus on Israel this week comes as Romney prepares to visit Jerusalem. The presumptive Republican nominee is a critic of Obama's policy toward Israel and has promised to ramp up US aid to the Jewish state, although Obama officials say the administration already provides record levels of funding.

A Romney spokeswoman said the former Massachusetts governor was happy to see steps being taken to enhance security cooperation with Israel.

"Unfortunately this bill does nothing to address yesterday's evasiveness from the White House on whether President Obama recognises Jerusalem as the capital of Israel, which raised doubt about the President's commitment to our closest ally in the region," said Romney spokeswoman Amanda Henneberg.

Romney is in London on the first leg of an overseas trip designed to burnish his foreign policy credentials.