KUFA, Iraq — U.S. troops and militiamen loyal to radical Shiite cleric Muqtada al-Sadr clashed near holy shrines in Karbala on Friday, while al-Sadr delivered a defiant sermon at a Kufa mosque. Gunmen killed two journalists from Polish television on the road to the troubled cities.

Despite the presence of hundreds of U.S. troops nearby on a mission to capture him, al-Sadr arrived from Najaf at the main mosque in Kufa surrounded by a large number of his heavily armed black-garbed gunmen, including at least one carrying an anti-aircraft gun.

"Yes, yes, to freedom! Yes, yes, to independence," several thousand worshippers chanted as the young renegade cleric delivered a sermon condemning the United States over the abuse of Iraqi detainees at Baghdad's Abu Ghraib prison.

"What sort of freedom and democracy can we expect from you when you take such joy in torturing Iraqi prisoners?" said al-Sadr, his shoulders draped with a white coffin shroud symbolizing his readiness for martyrdom.

U.S. troops have not moved to capture al-Sadr for the past four weeks as he has attended the Kufa prayers. The military has been treading carefully in its confrontation with al-Sadr and his al-Mahdi army militia, fearing that if it is too aggressive near some of Shiism's holiest shrines it will inflame Iraq's Shiite majority.

Still, U.S. forces have intensified the crackdown over the past week, with increasingly bloody clashes in several southern cities. On Friday, large explosions and gunfire were heard near the mosque that serves as al-Sadr's headquarters in central Karbala - about 500 yards from two major shrines. The shooting was heard soon before Friday noon prayers.

The exchange of fire followed hours of clashes in at least three other parts of the city that lasted from before dawn until midmorning. There was no immediate word on casualties from the day's skirmishes.

Two journalists from state-run Polish television, a Pole and an Algerian, were killed as they drove from Baghdad to Najaf when gunmen in another vehicle sprayed their car with gunfire.

A Polish cameraman was wounded in the arm in the shooting, said Adam Wielgosz, an official at the Polish Embassy in Baghdad.

The shooting took place near Mahmoudiyah, about 30 kilometres south of Baghdad and 57 kilometres north of Karbala - the site of several slaying of foreigners in past months. An American missionary and two CNN employees were killed in February and January in the same area.

Also, an audio recording attributed to Osama bin Laden offered rewards in gold for the killing of top U.S. and UN officials in Iraq or of the citizens of any nation fighting here.

The 20-minute recording, dated Thursday, appeared on two Web sites known for militant Islamic messages. The voice sounded like that of bin Laden and the words were heavy with Quranic verse, but the authenticity of the recording could not immediately be verified.

Al-Sadr's al-Mahdi army militia was out in force in Kufa and Najaf on Friday, fearing a U.S. assault to capture al-Sadr could be imminent. Militiamen and U.S. troops had a fierce mortar exchange overnight.

Fighters, draped with ammunition belts and carrying automatic weapons and rocket-propelled grenade launchers, moved around the streets of the two cities in large numbers and took positions behind earthen mounds and behind buildings.

But U.S. commanders suggested they would stick to their policy of not moving against him on Fridays out of respect for the weekly Islamic day of prayer.

In Kufa, al-Sadr's vehicle pulled up right next to the door of the main mosque, and the cleric quickly slipped in. For much of his sermon, al-Sadr denounced the abuse at Abu Ghraib, which has caused outrage across the Arab world.

Al-Sadr demanded guards who have been charged with abuse be handed over to Iraqi courts for trial and dismissed apologies from President Bush. "I tell this to Bush," al-Sadr said. "Your statements are not enough. They (the guards) must be punished in kind."

Overnight, a barrage of mortars hit the U.S. base in Najaf. The U.S. troops responded with heavy force, firing back with mortars and howitzers, while F-16s overhead directed fire.

The militiamen's mortar position was destroyed, said Maj. Todd Walsh, of the 2nd Battalion, 37th Regiment, 1st Armoured Division. With each night's fire, the fighters' mortars had become more accurate as they honed in on the base, so commanders decided to intensify their response to wipe out the position, Walsh said.

In Karbala on Thursday, coalition troops clashed with dozens of al-Sadr militiamen. A witness told Associated Press Television News that troops fired on the insurgents and destroyed four buses of Pakistani pilgrims, which were seen burning. The witness said "three or four" Pakistanis were killed.