(Adds Caldwell quotes on earlier abduction, background)

By Paul Tait

BAGHDAD, May 20 (Reuters) - Six U.S. soldiers and an interpreter were killed by a roadside bomb in western Baghdad on Saturday, the U.S. military said as a search for three abducted soldiers entered its second week on Sunday.

The soldiers and their interpreter had been uncovering caches of weapons, including grenades and small arms, as well as bomb-making equipment in the capital over the past week.

"Finding these weapons caches helps reduce the threat of insurgent attacks and continues to aid in providing a more secure and safe living environment for the Iraqi people," the military said in a statement.

More than 3,400 U.S. soldiers have been killed since the U.S.-led invasion to topple Saddam Hussein in 2003.

At least 70 U.S. troops have died so far this month as the U.S. military pours thousands of extra troops into the capital as part of a security crackdown seen as a last-ditch attempt to stop Iraq sliding into all-out sectarian civil war.

Fifteen soldiers have been killed in the past three days.

Thousands of U.S. troops are searching for three soldiers missing since an ambush south of Baghdad on May 12. Four U.S. soldiers and an Iraqi interpreter were killed in the attack.

An al Qaeda-linked group has claimed it has the soldiers. General David Petraeus, the commander of U.S. forces in Iraq, says he believes at least two of the soldiers are still alive.

The soldiers were abducted in a Sunni Arab insurgency stronghold known as the "triangle of death", where al Qaeda also abducted two U.S. soldiers last June. Their badly mutilated bodies were found days later after a similarly huge search.

Military spokesman Major-General William Caldwell told CNN that U.S. forces had on Friday killed the mastermind behind an audacious assault on a government compound in the holy Shi'ite city of Kerbala in January.

Insurgents posing as Americans had entered the compound, killed a U.S. soldier and drove off with four others whom they later shot and killed.

CRACKDOWN

The security crackdown began in mid-February as part of a plan to buy time for the government of Iraqi Prime Minister Nuri al-Maliki to meet a series of political benchmarks aimed at promoting national reconciliation.

The benchmarks, including a crucial revenue-sharing oil law, are aimed at drawing Sunni Arabs dominant under Saddam away from the insurgency and into the political process alongside majority Shi'ites and Kurds.

Almost a dozen mortar rounds could be heard landing in and around the heavily fortified Green Zone on Sunday, the most secure area in the capital which houses the Iraqi parliament, many government ministries and the U.S. embassy.

There was no early indication of any casualties from the latest episode of what has become a daily occurrence. It came a day after two mortar rounds landed there while Tony Blair was making an unannounced visit, his last as British Prime Minister.

Blair stressed that neighbouring countries should understand and respect that Iraq's future should be determined by Iraqis.

Ambassadors from the United States and Iran will meet in Iraq on May 28 to discuss security in the country, a rare meeting between the two bitter rivals.

Washington accuses Iran of fomenting violence in Iraq by backing Shi'ite militias there and of providing weapons and the technology for new versions of roadside bombs, by far the biggest killer of U.S. forces in Iraq.

Iran denies the charges and accuses the United States of igniting tension between Iraq's Shi'ites and Sunni Muslims.

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