US Congress has rushed through a $US225 million ($A243.44 million) bill to support Israel’s missile defence system, with House of Representatives approval coming just before lawmakers began a five-week summer break.

The money will go to restocking Israel’s Iron Dome, which has been credited with shooting down dozens of incoming rockets fired by Palestinian militants over three and a half weeks of war.

The vote came two days after the Pentagon announced ammunition deliveries to the Jewish state and as a planned 72-hour ceasefire between Israel and Hamas unravelled almost as quickly as it began.

The House’s 395-8 vote in favour followed Senate adoption of the legislation by voice vote earlier in the day.

The bill now goes to President Barack Obama for his signature.

At a White House news conference on Friday, Obama reiterated his support for Israel’s right to self-defence while urging greater protection for Palestinian civilians.

He cited Iron Dome as a concrete way the US is helping “make sure that Israel is able to protect its citizens.”

The Iron Dome system has emerged as a game-changer in the current round of violence, with Israeli officials citing a success rate as high as 90 per cent.

The system uses radar, advanced tracking technology and anti-missile batteries to follow the trajectory of an incoming rocket or mortar and determine if it is headed for a major population centre.

If an urban area is threatened, interceptors are fired to detonate in the air in close proximity to the missile.

Projectiles not posing a threat are allowed to fall in empty fields.

The system targets short-range rockets with a range between 3.2 kilometres and 72 kilometres. Interceptors cost as much $US100,000 apiece.

Shortly after the Gaza conflict erupted, Obama praised the missile defence systems as proof of America’s commitment to Israeli security.

His administration sent the Israeli request to Congress for more Iron Dome money even as the president and Secretary of State John Kerry have been seeking a humanitarian ceasefire.

Created by Israel’s Rafael Advanced Defense Systems, Iron Dome has enjoyed strong US technological and financial support.

Throughout its history, the US has provided more than $US700 million to help Israel cover costs for batteries, interceptors, production costs and maintenance, the Congressional Research Service said.

The total already appeared set to climb above $US1 billion after Senate appropriators doubled the Obama administration’s request for Iron Dome funding for the fiscal year beginning 2015.

Now it seems likely to rise even further, with Obama expected to sign any bill swiftly into law.

However, it’s unclear how quickly the new supplies might reach the battlefield. And Israel and Hamas may be in for a prolonged fight. A ceasefire arranged by the United States and United Nations collapsed shortly after it started on Friday morning.

More than 1400 Palestinians, mostly civilians and more than 60 Israelis, mostly soldiers, have been killed in the last 25 days.

Some of Congress’ Iron Dome money could go to US defence contractor Raytheon, which can manufacture components of the system after a March 2014 agreement between Israel and the United States.

The two companies also are collaborating on a system targeting mid-range rockets that can travel between 75 kilometres and 320 kilometres.

The aim of this is to protect Israel against Hezbollah in Lebanon and President Bashar Assad’s government and Sunni extremists in Syria.

With an eye on Iran, Israel is also developing a deterrent against longer-range threats. The next generation of the Arrow system is scheduled to deploy in 2016.