The operation, code-named “Juniper Cobra,” is taking place in the Negev desert in southern Israel with thousands of American and Israeli troops. Both sides described the timing as routine, denying a direct connection to the Iran threat.

Israeli and U.S. forces are testing new ways to deal with missiles capable of carrying nuclear, chemical and biological warheads, according to military officials who spoke on condition on anonymity because they were not authorized to give details of the operation.

The United States is Israel’s closest ally, providing about US$2.2 billion (euro1.65 billion) a year in military assistance and coordinating many aspects of defense policy.

Stewart Tuttle, the U.S. Embassy spokesman in Tel Aviv, would not directly say if the goal was to counter a potential nuclear threat.

“It is a computer simulation exercise designed to test the interoperability of the air defense system,” he said. “The air defense can protect you against whatever happens to be on the warhead.”

Both Israel and the U.S. have expressed concern about Iran’s nuclear program, suspecting that the goal is to produce nuclear weapons, despite Iranian denials. Israel considers Iran its most serious strategic threat, since its president has often called for Israel’s destruction and the nation has developed long range missiles.

This is the fourth time the two countries have conducted the joint “Juniper Cobra” exercise, held every two years. The current drill began last week and is set to end Tuesday.

During the last exercise, in 2005, the tests included integrating the Israeli-made Arrow anti-ballistic missiles with the U.S. Patriot system to create a multilayered air defense system.

The Arrow, a joint U.S.-Israeli project, is the world’s first operational anti ballistic missile system.

The concept is for the Arrow to intercept inbound missiles at high altitudes, while the Patriot provides cover at lower levels.

The U.S. deployed Patriot batteries in Israel in 1991, when Iraq fired 39 Scud missiles at Israel during the first Gulf war.