WASHINGTON, DUBAI, CAIRO - Israel launched a pre-emptive attack against Iran on Saturday, and a United States attack is underway, plunging the Middle East into a renewed military confrontation and further dimming hopes for a diplomatic solution to Tehran's nuclear dispute with the West.

The U.S. military initiated a series of strikes against targets in Iran, two U.S. officials told Reuters, speaking on condition of anonymity. The scope of the air and sea operations was not immediately clear. Iran was preparing a crushing retaliation, an Iranian official told Reuters.

A source told Reuters that Iran's supreme leader, Ayatollah Ali Khamenei, was not in Tehran and had been transferred to a secure location.

The attack follows a 12-day air war in June between Israel and Iran and repeated U.S.-Israeli warnings that they would strike again if Iran pressed ahead with its nuclear and ballistic missile programmes.

"The State of Israel launched a pre-emptive attack against Iran to remove threats to the State of Israel," Defence Minister Israel Katz said.

MONTHS OF PLANNING WITH US, ISRAEL SAYS

An Israeli defence official said the operation had been planned for months in coordination with Washington, and that the launch date was decided weeks ago.

Explosions were heard in Tehran on Saturday, Iranian media reported, and sirens sounded across Israel around 8.15 a.m. local time in what the military said was a proactive alert to prepare the public for the possibility of an incoming missile strike.

The Israeli military announced the closure of schools and workplaces, with exceptions for essential sectors, and a ban on public airspace. Israel closed its airspace to civilian flights, and the airports authority asked the public not to go to any of the country's airports.

The U.S. and Iran renewed negotiations in February to try to resolve the decades-long dispute through diplomacy and avert the threat of a military confrontation that could destabilise the region.

Israel, however, insisted that any U.S. deal with Iran must include the dismantling of Tehran's nuclear infrastructure, not just stopping the enrichment process, and lobbied Washington to include restrictions on Iran's missile programme in the talks.

Iran said it was prepared to discuss curbs on its nuclear programme in exchange for lifting sanctions but ruled out linking the issue to missiles.

Tehran also said it would defend itself against any attack.

It warned neighboring countries hosting U.S. troops that it would retaliate against American bases if Washington struck Iran.

In June, the U.S. joined an Israeli military campaign against Iranian nuclear installations, in the most direct American military action ever against the Islamic Republic.

Tehran retaliated then by launching missiles toward the U.S. Al Udeid air base in Qatar, the largest in the Middle East.

Western powers have warned that Iran's ballistic missile project threatens regional stability and could deliver nuclear weapons if developed. Tehran denies seeking atomic bombs.

- Reuters bureaux; Editing by Michael Georgy and Barbara Lewis