JERUSALEM — The Israeli military says its largest ever exercise with the United States has begun.

A major air defense drill called Austere Challenge 2012 started Sunday and includes more than 3,500 Americans and 1,000 Israelis. The militaries are practicing to work together against a range of threats facing Israel, the main U.S. ally in the Mideast.

The military exercise was originally scheduled for April but was postponed at Israel's request. No reason was given but it came amid growing talk of Israel preparing to attack Iran if Tehran does not cease its uranium enrichment program.

The weeks-long joint exercise will test multiple Israeli and American air defense systems against incoming missiles and rockets from places as far away as Iran.