Amid growing fears Israeli leaders are preparing a pre-emptive attack on Iran's nuclear facilities, defense sources say the Arrow-3 interceptor, seen as Israel's ace in the hole against Tehran's ballistic missiles, is close to its first flight test.

The two-stage missile being developed by state-owned Israel Aerospace Industries and the Boeing Co. of the United States will be Israel's main line of defense against Iran's growing arsenal of immediate-range Shehab-3 missiles and the more advanced Sejjil-2 weapons under development.

The upcoming test-firing, delayed for about a year, will take place against a backdrop of growing threats of retaliation by Tehran if Israel, or even the United States currently locked in its own confrontation with Iran in the Persian Gulf, attacks the Islamic Republic.

The Arrow-3, the most advanced component of a multilayered missile defense shield the Israelis are building, will take place "soon," says Itzhak Kaya, who heads the Arrow program.

This will be the first test of all the Arrow-3 systems. Subsystems have already been tested.

The second stage has its own propulsion unit that enables it to maneuver toward its target. It can reach twice the altitude of Arrow-2.

The Pentagon, which provides much of the funding for the joint program and has been seeking to persuade U.S. legislators that it's worth Congress investing taxpayers' money in the project, says Arrow-3 will be able to provide four times the coverage of Arrow-2.

Kaya disclosed that recent testing involved simulated interceptions to evaluate Arrow-3's detection capabilities.

The new variant operates with an advanced version of the EL/M-2080 Green Pine solid-state, phased array radar system manufactured for Arrow by Elta Electronic Industries of Ashdod, a subsidiary of IAI's Electronic Group.

"A successful identification of the attacking missile by the Arrow System increases the chances and certainty of an interception," Kaya said.

Neither of the first two Arrow variants has been used on combat and there have been concerns about its ability to counter a heavy salvo of Shehab or Sejjil missiles.

Uzi Rubin, considered one of the pre-eminent missile system analysts in the Middle East, recently said Arrow could cope with any missile fired by the Iranians.

"I can't say that every incoming will be known down," he told Israel Army Radio. "There isn't 100 percent protection and not everything is a success.

"But for every single missile coming from Iran there's a single Arrow missile capable of intercepting it one for one."

Rubin, a former air force brigadier general, was head of Israel's Missile Defense Organization in 1991-99 and oversaw development of the Arrow series.

"Iran has between 300 and 400 Shehab-3 missiles it can fire at Israel," he said.

He also disclosed that Iran's aerospace industries manufacturing the Shehab, a program controlled by the Islamic Revolutionary Guard Corps, has significantly improved the missile's accuracy from "a marked target that could cover a few kilometers to just a few hundred meters."

That would make the Iranian missiles a much greater threat to Israeli airbases and military installations, as well as the national infrastructure, than previously thought.

The latest variant of the high-altitude, long-range Arrow is designed to intercept incoming ballistic missiles in space outside the Earth's atmosphere in the final phase of their trajectory and destroy them on impact.

The Arrow-2, the version operationally deployed by the Jewish state, is built to tackle hostile missiles at lower altitudes within the atmosphere by exploding near them.

The first Arrow missiles were deployed in 2000. There are at least two batteries operational, one in northern Israel and the other outside of the coastal Palmachim air force base south of Tel Aviv where most of the program's test flights have been conducted.

Arrow-2 will remain as a secondary line of defense, with two other systems designed to counter shorter-range missiles and rockets closer to the ground.

The Iron Dome system, developed to intercept projectiles with a range 5-40 miles, has been in action against Palestinian Grad and Qassem rockets since March 2011 and is reported to have a kill rate of around 75 percent.

It's built by Rafael Advanced Defense Systems, which is also developing the David's Sling system to counter missiles with a range of up to 130 miles. It's not expected to be deployed for another 18 months.