The United States and Israel are set to launch a major military exercise in a show of unity aimed at Iran, despite friction between American and Israeli leaders over how to counter Tehran's nuclear ambitions.

The air defense drills, dubbed "Austere Challenge 2012," will unfold later this month and last about three weeks, with 3,500 US troops and 1,000 Israeli forces taking part, officers said Wednesday.

"This is the largest exercise in the history of the longstanding military relationship between the US and Israel," said Lieutenant General Craig Franklin, 3rd Air Force Commander, who is overseeing the drill along with his Israeli counterpart, Brigadier General Nitzan Nuriel.

"This exercise will improve the cooperative missile defense of Israel and will promote regional stability and help ensure a military edge," Franklin told reporters in a teleconference.

But the drill is about more than missile defenses.

The elaborate exercise takes place at a politically-charged moment, amid speculation about a possible Israeli pre-emptive attack on Iran, a hotly-contested US presidential election weeks away and parliamentary polls expected in Israel within a few months.

The drill's "scenario is to deal with threats from all fronts," Nuriel, the Israeli commander, told the same phone conference.

"Anybody can get any type of message he wants from this exercise. The fact we are practicing together and working together is a strong message by itself."

Although Israel faces rocket attacks out of Gaza and missile threats from Syria and Hezbollah militants in Lebanon, the main worry for the Jewish state is Iran's growing arsenal of ballistic missiles.

In a report this year to Congress, the Pentagon warned that Iran's missiles could hit Israel and Eastern European countries, including an extended-range version of the Shahab-3 and a medium-range ballistic missile with a range of 2,000 kilometers.

The missile threat, combined with the crisis over Iran's disputed nuclear program, prompted Israeli authorities in August to test a SMS public alert system designed to warn the population of an imminent attack.

In the works for two years, the joint exercise originally was scheduled for April but was postponed at Israel's request, without an official explanation.

The drill highlights US-Israeli cooperation just as President Barack Obama seeks to deflect criticism from his Republican rival Mitt Romney, who has accused him of neglecting America's ally and rebuffing Israeli Prime Minister Benjamin Netanyahu.

Obama and Netanyahu disagree over the urgency of the perceived threat posed by Iran, with the US president favoring more time for diplomacy and international sanctions to rein in Tehran's uranium enrichment work.

Netanyahu has publicly aired his differences with the Obama administration over the Iran issue, scolding Washington for failing to set its own "red lines" that would trigger military action against Tehran.

The West believes Iran is secretly pursuing plans to develop a nuclear weapons capability, but Tehran insists its program is designed purely for peaceful purposes to generate electricity.