The Basra police commander on Friday said the roadside bomb used in an attack that killed four British soldiers had not been used in southern Iraq before, and his description of the deadly weapon indicated it was a feared Iranian-designed explosively formed projectile.

Anbar province has been a stronghold of the Sunni insurgency but many tribes in the region recently switched allegiance, with large numbers of military-age men joining the police force and Iraqi army in a bid to expel Al Qaeda in Iraq fighters.

The U.S. military has claimed Iran is supplying Shiite militia fighters in Iraq with explosively formed projectiles, known as an EFP. They hurl a molten, fist-sized copper slug capable of piercing armored vehicles.

The four British soldiers — including two women — were killed Thursday as the American military announced the deaths of eight more U.S. soldiers since Tuesday.

The Basra region police commander, Maj. Gen. Mohammed al-Moussawi, said two similar bombs had been discovered Friday morning; one was discovered on the road leading to Basra Palace, the compound that houses a British base and the British and U.S. consulates. A second was uncovered in the western Hayaniyah district where Thursday's attack occurred. The area is known as a stronghold of the Mahdi Army, a militia loyal to radical Shiite cleric Muqtada al-Sadr.

The reported deaths of the American forces and the bomb attack on the British unit marked the start of the eighth week of the joint U.S.-Iraqi security crackdown in Baghdad and surrounding territory.

Also Thursday, the U.S. military confirmed an American helicopter carrying nine people had been downed south of Baghdad and that four were injured.

An Iraqi army official, speaking on condition of anonymity because of security concerns, said the helicopter went down after it came under fire from anti-aircraft guns near the Sunni insurgent stronghold of Latifiyah, 20 miles south of Baghdad. The U.S. military did not confirm that account.

It was the ninth U.S. helicopter to go down in Iraq this year. The U.S. military has studied new evasive techniques, fearing insurgents have acquired more sophisticated weapons or have figured out how to use their arms in new and effective ways.

Prime Minister Tony Blair called the Basra attack an "act of terrorism" and suggested it may have been the work of militiamen linked to Iran. He stopped short of accusing Tehran, however.

"Now it is far too early to say that the particular terrorist act that killed our forces was an act committed by terrorists that were backed by any elements of the Iranian regime, so I make no allegation in respect of that particular incident," Blair said.

He added, however, "This is maybe the right moment to reflect on our relationship with Iran."

One U.S. soldier died and two were wounded in a roadside bombing Thursday in restive Diyala province north of Baghdad, the military said. Four others died Wednesday in two roadside bomb explosions in southern Baghdad and north of the capital, while a fifth was killed by small-arms fire in the eastern part of the city. Two other soldiers were killed by small-arms fire on Tuesday — one in eastern Baghdad and another on foot patrol in the southern outskirts of the capital.

The deadly attack against the British patrol in southern Iraq was the greatest loss of life for Britain in more than four months and it cast a shadow over celebrations marking the return of 15 British sailors seized by Iran two weeks ago in disputed waters in the Persian Gulf.

"Just as we rejoice at the return of our 15 service personnel so today we are also grieving and mourning for the loss of our soldiers in Basra, who were killed as the result of a terrorist act," Blair said.

The British patrol struck a roadside bomb and was hit by small-arms fire early Thursday in the southern city of Basra, British military spokeswoman Capt. Katie Brown said. The explosion created a 9-foot crater in the road. Hours after the attack, a British soldier's helmet was still laying in the street among dozens of spent bullets.

The latest casualties raised to 140 the number of British forces to die in Iraq since the March 2003 invasion — 109 in combat.

Blair has announced that Britain will withdraw about 1,600 troops from Iraq over the next few months and hopes to make other cuts to its 7,100-strong contingent by late summer.