The US military has launched two big operations west and north of Baghdad this week to try to root out what it says are diehard Saddam Hussein loyalists behind a recent spate of attacks on American troops in mainly Sunni Muslim areas.

A US statement said an organised group of fighters had fired rocket-propelled grenades at a 4th Infantry Division tank patrol in Balad, about 60 miles from the capital.

"The tanks returned fire, killing four of the attackers, and forcing the remainder to flee," it said. "Tanks and Bradley fighting vehicles reinforced with AH-64 Apache helicopters pursued the enemy personnel, killing 23 of the attackers."

No US casualties were reported in the clash.

Some 4,000 troops have been scouring an area around the Tigris river northeast of Balad since Monday in "Operation Peninsula Strike," which the military said was the biggest operation it had launched in the past six weeks.

In the other big assault, launched early on Thursday, at least 70 people were killed at a "terrorist" training camp in northwest Iraq, a US military spokesman said on Friday.

He said the 101st Airborne Division and special operations units were involved in the raid that began with an air strike on the camp, 90 miles northwest of Baghdad. One US soldier was wounded. The operation was still in progress.

The spokesman said a US helicopter had been shot down during the operation on Thursday. The Apache's two-member crew were rescued unhurt as two other Apaches engaged irregular Iraqi fighters. It was the first time a US helicopter had been shot down since the fall of Baghdad on April 9.

The statement said 70 to 80 SAM-7 surface-to-air missiles, 75 to 78 rocket-propelled grenades and a score of AK-47 assault rifles had been found at the suspected training camp.

US officials have released no other details on the camp, though one said the military believed that some of those present were not Iraqis. Arab volunteers from several countries fought with Iraqi forces during the war launched on March 20.

AFP adds from Makhoul: Fires blazed on a major pipeline from Iraq's northern oilfields yesterday after what residents said were twin bomb attacks aimed at sabotaging exports through Turkey.

An AFP correspondent saw two separate fires on the pipeline, 15 km from the key refinery town of Baiji, close to the main highway between Baghdad and the northern regional capital of Mosul.

US military helicopters hovered overhead.

Residents questioned by the news agency at the nearby Al-Amin coffee shop said the pipeline had been attacked by Iraqis using explosives around 8:45 pm localtime Thursday, the same day Iraq awarded its first post-war oil export contracts.

"We heard two explosions and ran," said the coffee shop's owner, Abu Ala. "We saw fire shooting out of the pipeline in two places. Shortly afterwards two American helicopters arrived."

Customers said it had been a deliberate act of sabotage.

"Some Iraqis came and blew it up," said Kazem Ibrahim.

"It's to stop the Americans taking the oil out to Turkey," said Khidr Aziz.

Some 225 km north of Baghdad, Baiji is home to a major refinery and power station, processing fuel from the oilfields around the northern city of Kirkuk.

Less than an hour's drive north of Saddam Hussein's native city of Tikrit, the mainly Sunni Arab region around Baiji was considered a stronghold of his Sunni-dominated regime and several residents expressed hostility to the US-led occupation.

"The Iraqis won't change. If Saddam disappears, there will be 20 others emerging every day," said Hussein Abu Ali.

On Thursday, Iraq's US-led administration awarded a raft of contracts to international oil companies to lift crude, the first since the US-led war.

A coalition spokesman said the contracts were for exports from the southern oilfields around Basra and the lifting of Kirkuk crude already in storage at the Turkish Mediterranean port of Ceyhan at the other end of Iraq's northern export pipeline.

Meanwhile, US troops detained 74 suspected Al-Qaeda sympathisers in a raid in northern Iraq, Central Command said yesterday in its first explicit claim of involvement by Osama bin Laden's Islamist militant network in post-war unrest here.

"The 173rd Airborne Brigade conducted a raid Thursday near (the northern oil capital of) Kirkuk after receiving intelligence information about alleged anti-coalition elements. They apprehended 74 suspected Al-Qaeda sympathisers," Centcom said in a short statement.

US commanders have thus far put the main blame for deadly attacks against coalition troops here on loyalists of the ousted regime of Saddam Hussein although they have acknowledged that foreign "fanatics" might also have had a role.

Reuters adds: The main oil export pipeline linking Iraq and Turkey was hit by fire and explosions due to a gas leak, US authorities in Iraq said yesterday.

Turkey said investigations were under way to establish whether sabotage was to blame for the blast at the Iraqi section of the pipeline in northern Iraq late on Thursday.

US military spokesmen said there was no sign that sabotage had caused the explosions, which comes after occupying US forces have blamed saboteurs for undermining their efforts to restore the oil industry.

"There is no evidence that there was any hostile intent. US and Iraqi engineers are assessing the extent of the damage," a spokesman told Reuters.