The Pentagon is expected to announce an additional $680 million in U.S. aid for Israel to boost the development of anti-missile systems to shield the Jewish state from a feared bombardment by Iran, Syria, Hezbollah and the Palestinians.

The announcement will coincide with the visit to Washington of Israeli Defense Minister Ehud Barak, who arrived Monday.

The funds will be allocated to the Iron Dome counter-rocket system developed and built by Rafael Advanced Defense Systems.

Assistance for two other systems -- the high-altitude Arrow 3 built by state-owned Israel Aerospace Industries and designed to intercept Iranian and Syrian ballistic missiles outside the Earth's atmosphere, and David's Sling, another Rafael system intended to counter medium-range missiles -- has already been approved.

Arrow is to get $119 million, with David's Sling, also known as Magic Wand, will receive $165 million.

All told, that adds up to $948.74 million in U.S. aid for Israel's drive to construct a unique four-layer missile shield that will become fully operational in the next 3-4 years and is capable of combating everything from short-range unguided rockets to highly accurate intermediate ballistic missiles.

The move follows intense bipartisan pressure in the U.S. Congress on U.S. President Barack Obama's administration to aid Israel as it confronts Iran and its Arab foes amid rising tensions in the Persian Gulf, a revolution in Syria and growing hostility in Egypt where an Islamist surge threatens its historic 1979 peace treaty with the Jewish state.

The new U.S. funding is the largest single military aid grant authorized for Israel or any other ally.

It's in addition to the $3 billion a U.S. military aid the Israelis get every year and dwarfs the $205 million Washington provided in fiscal 2010 to fund the Iron Dome anti-rocket system that's already been in action against Palestinian attacks.

It also coincides with heavy U.S. pressure on Israeli Prime Minister Binyamin Netanyahu's coalition government to hold back on threatened pre-emptive strikes against Iran's nuclear facilities.

Washington says that assault would trigger a Middle Eastern conflict with the potential to cause immense destruction across the Middle East. That's the last thing the Obama administration wants in a presidential election year as it prepares to withdraw forces from Afghanistan as it already has in Iraq.

The scale of the new aid package, and its timing, has raised suggestions that Obama is seeking to buy off Netanyahu, even though the U.S. president is at considerable odds with the hawkish Israeli leader over his refusal to halt Jewish settlement activity in the occupied West Bank to revive the fading peace process.

Indeed, since Obama first denounced Israel's settlement program in 2009, in terms far harsher than his predecessors, Israel has accelerated settlement expansion.

Obama's differences with Netanyahu could cost him heavily in November but the new military aid package, just shy of $1 billion, could secure a big chunk of the Jewish vote.

The Congressional Research Service said Israel is the largest cumulative recipient of U.S. foreign assistance since World War II. As of 2012, the United States has provided Israel with $115 billion in bilateral assistance.

The U.S. aid may turn out to have strings attached. Israel's Globes business daily reports that Congress' Strategic Forces Subcommittee of the House Armed Services Committee made the new grant conditional on the United States receiving rights to Iron Dome's unique technology.

It also wants part of the production to be transferred to U.S. companies, suggesting this could involve job losses in Israel.

"The legislators want the head of the U.S. Missile Defense Agency, Lt. Gen. Patrick O'Reilly, to 'ensure the United States has appropriate rights to this technology,' before the money is transferred to Israel," Globes reported.

The wording there is pretty vague, and Globes observed that the Israeli assessment is "the U.S. legislators left broad room for maneuver for the Defense Department in its negotiations with the Israeli Ministry of Defense."

The plan is to bolster Israeli efforts to construct a multilayered defense shield to protect its cities and strategic installations against an anticipated missile bombardment that Israeli strategists say Iran and its Arab allies could sustain for many weeks.

The funding was actively encouraged in Congress by the Defense Department, informed sources say. The House Armed Services Committee last week authorized the $680 million.