The United States military says it has detected a significant increase in sophisticated roadside bombs in Iraq and believes that orders to send components for them came from the “highest levels” of the Iranian goverment, a senior intelligence officer said today.

The officer, briefing reporters on condition of anonymity, said that that between June 2004 and last week, more than 170 Americans had been killed by the bombs, referred to by the military as “explosively formed projectiles" (EFPs) and capable of destroying an Abrams tank.

The officer said American intelligence analysts believe the EFPs are manufactured in Iran and smuggled into Iraq on orders from the top of the Iranian government.

American officials have long claimed that weapons were entering Iraq from Iran but had stopped short of alleging involvement by top Iranian leaders.

The US officer said Iran was working through “multiple surrogates”, mainly “rogue elements” of the Shia Mahdi Army, to smuggle the EFPs into Iraq. He said most of the components are entering the country at crossing points near Amarah, the Iranian border city of Meran and the Basra area of southern Iraq.

American authorities had briefed Iraq’s Shia-led government on Iran’s involvement and Iraqi officials had asked the Iranians to stop, the officer said. Prime Minister Nouri al-Maliki, a Shia, has said he told both America and Iran that he does not want his country turned into a proxy battlefield.

Last week, US officials said they were investigating allegations that Jamal Jaafar Mohammed, a Shia elected to the Iraqi parliament in December 2005, was a main conduit for Iranian weapons entering the country. Mohammed is believed to have fled to Iran.

Today's allegations were made in a briefing which had been set for last week but was postponed so that the Pentagon could review the information to make sure it was accurate, US defence officials said.

During the briefing, the officer said that one of the six Iranians detained in January in the northern city of Irbil was the operational commander of the Quds Brigade, a unit of the Iranian Revolutionary Guards that trains and equips Shia militants abroad. He was identified as Mohsin Chizari, who was apprehended after slipping back into Iraq after a ten-month absence, the officer said.

The Iranians were caught trying to flush documents down a lavatory, the American officer said. They had also tried to change their appearance because bags of their hair were found during the raid, he added.

The dates of manufacture on weapons found so far indicate they were made after fall of Saddam Hussein, mostly in 2006, and the "machining” on the components is traceable to Iran, the officer claimed. There is no evidence, US officials told reporters, that Iranian weapons were behind the latest spate of helicopter crashes.

Major General Jim Simmons, the deputy commander of Multinational Corps-Iraq, said that since December 2004, American pilots have been shot at on average about 100 times a month and in most cases the rounds do little damage.

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