But the Pentagon said it was unaware of any fatal attacks on US troops in Iraq Tuesday, reporting instead that six US soldiers were injured in two separate bombing attacks in the Baghdad area.

In the first attack, at around 8:45 am (0445 GMT) just southwest of Baghdad, three soldiers were hurt when their convoy was hit by "an improvised explosive device," said Pentagon spokeswoman Diane Perry.

The second attack, also on a convoy of US soldiers, occurred about one hour later in central Baghdad. Three soldiers were wounded in that attack, which US Defense Department officials said also used an unspecified "explosive device."

Perry said two Iraqis were also injured in the attacks, one of whom later died in hospital.

According to witnesses, four US soldiers were killed and two others wounded Tuesday in a rocket-propelled grenade (RPG) attack on their vehicle by unknown assailants in central Baghdad.

The attack occurred at 10:00 am (0600 GMT), when assailants fired an RPG at a US Humvee light multi-wheeled vehicle near a gas station in the Al-Mustansiriya neighborhood, they said.

US forces also faced charges that explosions at a mosque in the flashpoint town of Fallujah west of the capital were caused by a US airstrike.

But the US military said in a statement that "US forces had no involvement with an explosion, which destroyed a building in a mosque courtyard."

US troops in the area "responded to the scene after notification from a US aircraft that spotted the explosion. The incident is being investigated by the Fallujah police and coalition forces," it said.

Another US spokesman said he believed "something like an ammunition dump" had exploded next to the mosque.

He added, asking not to be named, that coalition forces had been targeted in 10 separate attacks since Saturday.

US troops in Fallujah again came under fire at dawn Tuesday, in the fifth consecutive day of anti-US strikes in the town, while a military vehicle was burned out by unknown attackers in Yusufiyeh, south of the capital.

Witnesses said three explosions in Fallujah, including one at Al-Hassan mosque in the Al-Askari neighborhood, killed six Iraqis, believed to be theology students, and injured 15 late Monday.

Several eyewitnesses said the mosque had been the target of a US airstrike, as an aircraft was heard overflying the town before the shrine was hit by what they said was a missile.

Among the wounded was Sheikh Laith, the imam of the mosque, whose leg was amputated, the witnesses said.

Fallujah, 50 kilometres (30 miles) west of Baghdad, has been tense since US troops shot dead at least 16 people during protests there in late April.

In a separate incident on the highway near Yusufiyeh, about 20 kilometres (13 miles) south of Baghdad, two US soldiers were injured when their vehicle plunged into a deep hole in the road, witnesses said.

While some of them said attackers then fired an RPG at a second vehicle that had come to assist, others said Iraqis had poured gasoline over it and set it on fire.

In Washington, officials said the attacks were expected to continue despite aggressive action to stop them, while insisting that efforts to pacify and stabilise Iraq were on track and not getting bogged down.

At least 21 US and six British soldiers have now been killed as a result of hostile action since the war was declared effectively over two months ago, on May 1.

US officials have struggled to defeat perceptions the occupation of Iraq has stalled amid almost daily attacks on coalition troops and the slow progress of reconstruction efforts.

US overseer Paul Bremer said in his weekly press conference on Tuesday that "we are well on track to establishing an Iraqi interim administration by mid-July, as I have all along suggested."

Those efforts were again hit when Grand Ayatollah Ali al-Sistani, considered a "moderate", slammed the US-led coalition's plans to appoint a body to draft a new constitution, saying the occupation forces were not authorised to do so.

The Shiite cleric's edict said the plan would have to be preceded by a general election.

That is something the US-led authority has said it does not expect for at least a year while it attempts to wipe out resistance from the former regime, restore order and get the country's economy back on track.

The embarrassment was compounded by the arrest in the southern city of Najaf of the US-appointed interim governor, in a setback to the coalition's efforts to replace the ousted Baath Party system with accountable public officials.

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