US troops and equipment have begun arriving in Israel ahead of what a senior air force officer on Wednesday called "the largest exercise in the history of the longstanding military relationship between the US and Israel."

He did not give precise dates, saying only that it would begin towards the end of October or early November and last "about three weeks."

Lieutenant General Craig Franklin told journalists in a telephone briefing that the drill, "Austere Challenge 2012" (AC12) was defensive and unrelated to the Iran nuclear crisis, other Middle East developments or elections in the United States and Israel.

"While the scenario is driven by the overall situation in the Middle East, AC12 is not related to any specific current event," he said. "AC12 is not related to national elections nor to any perceived tensions in the Middle East."

Originally scheduled for spring, when the massive joint missile defence drill was postponed local media suggested that it could be a victim of disagreement between US President Barack Obama and Israeli Prime Minister Benjamin Netanyahu over a possible military strike on Iran.

It was also reportedly downsized, although Franklin said changes were minimal.

"Overall the scale of the exercise and the number of forces taking part in it is essentially unchanged," he said. "The total number of participants is the same, there's just a reduced US presence in Israel."

"The numbers changed a little bit, mainly on logistics and other support," Brigadier General Nitzan Nuriel, the chief Israeli planner for the manoeuvres, added.

"On troops on the ground there is no change and we are going to practise as we planned."

Despite disclaimers, the exercise, involving a total of 3,500 US personnel and 1,000 Israeli troops, will be seen as sending a clear signal to Tehran amid tension over Iran's nuclear drive, which Israel, Washington and much of the international community believe masks a weapons drive.

"Anybody can get any kind of message he wants from this exercise," Nuriel said.

"The fact that we are practising together, working together, is a strong message by itself," he added. "Anyone can take any lessons he wants from this exercise.

Franklin said: "It will promote regional stability and help ensure a military edge."

Of the 3,500 US personnel, 1,000 will be in Israel and the rest in Europe and the Mediterranean, Franklin said. Troops will train together on Israel's Iron Dome missile defence system, the latest version of the US Patriot and the Arrow anti-ballistic missile system, jointly developed by the two allies.

Command and control functions will be provide by a US Navy Aegis cruiser.

The total cost is around $38 million, with the United States' share at $30 million.