"It was a crude oil pipeline going from Kirkuk to Baiji," Majid Mamuni, general director in charge of pipelines at the Baiji refinery, told AFP.

"I think it was sabotage," he added.

Police and witnesses at the scene said a large explosion rocked Baiji Thursday night, while a fire along a section of pipeline was still raging Friday near the refinery town.

"It is an attack, a sabotage," said a high-ranking Baiji police officer who did not want to be identified. "It could be the Fedayeen (pro-Saddam Hussein militia), or supporters of the old regime, or criminals."

The police officer had earlier mistakenly identified the stricken line as a gas pipeline, Mamuni said.

Ayed Ayed Abdullah, a resident of Tal Abu Jarad village a few hundred metres from the scene, said he heard what "sounded like a bomb" as he was preparing for Thursday night prayers.

Baiji is about 200 kilometres north of Baghdad and is a vital hub in the strategic network of oil pipelines which criss-cross Iraq.

Meanwhile, four Iraqi men were killed yesterday and three US troops were lightly wounded in separate attacks against American forces near the town of Fallujah, amid a rise in violence in the flashpoint region, the US military said.

The Iraqis died in a rocket-propelled grenade (RPG) assault on a US convoy.

"Four Iraqis were killed and nobody from the American side was injured" in the morning attack, Sergeant Keith O'Donnell told AFP.

It was not immediately clear if the RPGs missed their mark or soldiers inside their vehicles were unhurt in the explosions.

US forces arrested nine people in the clash, seven of whom O'Donnell said were carrying grenades while two were found with suspect documents bearing photographs of ousted president Saddam Hussein.

He said eight attackers ambushed the Americans as they patrolled Fallujah's outskirts on reconnaissance.

Witnesses at the scene, in the village of Albu Alwan seven kilometres west of Fallujah, earlier said two Iraqis had been killed while an undetermined number of US soldiers were wounded during the attack and a subsequent gunbattle.

"I was arrested for two hours by American forces and I saw 12 (US) soldiers on the ground," said Majid Ibrahim Allawi, adding that the gunbattle lasted 90 minutes.

"They were driven in the direction of camp al-Habaniya," a former base of the Iraqi army now used by US forces.

O'Donnell said the clash was the latest in a growing number of attacks in the region considered a stronghold of resistance to the US-led occupation.

"It was one of eight attacks in the last 24 hours west of Baghdad, the most extensive attacks in a while," O'Donnell said.

In a separate incident, three US soldiers were lightly wounded when their vehicle was struck by a mine blast near the Habaniya base.

"Three Americans were slightly injured by a mine at 9:45 this morning near al-Habaniya," O'Donnell said. The extent of their injuries was not immediately known.

And at 10:30 am, an AFP correspondent witnessed an explosion on the road in Albu Alwan which occurred 15 metres from a US patrol, but there were no casualties.

Anit-US sentiment has run high in Fallujah, a Sunni Muslim bastion 50 kilometers west of Baghdad, ever since US troops shot dead at least 16 demonstrators in April.

A US special task force has been operating in the region hunting for Saddam, whose ability to elude capture since his regime was toppled April 9 has been seen as a spur for continued attacks on the US-led coalition.