Insurgents loyal to radical cleric Muqtada al-Sadr (search) downed a U.S. helicopter during fierce clashes Thursday in the holy city of Najaf (search), and the wounded crew was evacuated.

Also Thursday, a homicide car bombing killed five people and wounded 27 at a police station south of Baghdad, the Interior Ministry said.

The fighting in Najaf began early in the morning when al-Sadr's Mahdi Army (search) attacked a police station on Revolution of 1920 Square with mortars, rocket-propelled grenades and Kalashnikov rifles. Najaf's governor, Adnan al-Zurufi, told the Al-Jazeera satellite channel that the U.S. Marines intervened "to help the policemen protect the police stations and the city."

Al-Zurufi said armed militias attacked the police station at about 1:50 a.m. and two people had been killed so far. During the battle, a UH-1 helicopter was hit and crashed. The crew was hurt and evacuated to safety.

The Najaf fighting, the fiercest in weeks, threatens a fragile cease-fire between the Mahdi Army and Iraqi and U.S. authorities. Al-Zurufi warned of "very bad consequences" if the militiamen did not disarm and leave the holy city.

In the vehicle bombing in Mahawil, 53 miles south of Baghdad, a bus approached, two gunmen clad in police uniforms hopped out and opened fire on the police station. They escaped, while the bomber inside the bus died in the bomb explosion.

The blast damaged the gate of the station and a dozen nearby cars and left a crater in the ground.

"I was outside the building when I saw a car heading toward us. We started shooting. I'm sure we shot him but he managed to explode the car," said police Capt. Adel Omran, who has shrapnel in his leg said.

Insurgents have repeatedly targeted police as part of their campaign to destabilize the interim government — killing 710 from April 2003 to May 2004. The guerrillas see police as collaborators with the American-led coalition forces.

"What do these criminals want from Iraqis? They sometimes target the Sunnis, the Shiites and the Christians and other times they target the police and the army. They, however do nothing to the Americans," said Zayd Hadi, a civilian who was outside the station and suffered wounds to his face and stomach.

Farther north, a series of battles between Iraqi authorities and insurgents in the city of Mosul killed 14 civilians and eight insurgents Wednesday, the U.S. military said. Iraqi authorities said Thursday that 17 people had been killed and 47 wounded.

Iraqi authorities clamped a curfew on the area and sealed off bridges into the city to restore order. The fighting was the fiercest in Mosul in months, and local authorities said insurgents appeared to be testing the police. No Iraqi or coalition forces were killed in the violence, the U.S. military said.

Two of the militants killed included a member of the Al Qaeda-linked militant group Ansar al-Islam and a cousin of the group's founder, said Sarkawt Hassan, security chief in the Kurdish province of Sulaimaniyah.

The body of Sayed Omar Omar Mohammed, also known as Sayed Qutb, the cousin of founder Mullah Krekar, was found in a car in al-Yarmouk area in Mosul, Hassan said.

In the southern city of Basra, militants loyal to al-Sadr threatened Thursday to attack British forces in the area unless they freed four men detained in a raid on al-Sadr's party's office in Basra two days before.

"Otherwise the Mahdi army will confront the British forces, enter the city and take over important government buildings," said Salam al-Maliki, a spokesman for al-Sadr's Mahdi army militia.

The British said they hadn't received a formal ultimatum, "only rhetoric," said Maj. Ian Clooney. He said the men had been detained for further questioning, and did not elaborate.

On Tuesday, police said that al-Sadr's militias had kidnapped police officers apparently to use as leverage to force authorities to release militants being detained. His group denied the accusations, saying police were provoking al-Sadr's supporters by trying to arrest some of the group's leaders.

Insurgents have kidnapped scores of foreign hostages to force foreign companies and coalition troops from Iraq. In an effort to save the hostages, several companies have said they would stop their work here, and last month the Philippines withdrew its 51-member troop contingent to secure the freedom of a Filipino truck driver.

In a move to show kidnappers that none of the 31 other countries in the coalition would follow suit, the United States issued a statement Wednesday vowing not to make concessions to hostage-takers. Many of the other coalition members were expected to issue similar statements in the coming days, State Department spokesman Richard Boucher said.

"We are united in our resolve to make no concessions to terrorists," read the statement. "We understand that conceding to terrorists will only endanger all members of the multinational force, as well as other countries who are contributing to Iraqi reconstruction and humanitarian assistance," it said.