President Barack Obama will release an additional $70 million in military aid for Israel, a previously announced move that appeared timed to upstage Republican presidential rival Mitt Romney's trip to Israel this weekend.

The stepped-up aid, first announced in May and reiterated on Friday, will 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.

Obama announced the new military assistance as he signed a bill in the Oval Office expanding military and civilian co-operation 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, where he will meet Prime Minister Binyamin Netanyahu, with whom Obama has had frosty relations.