FALLUJAH, Iraq - U.S. forces today regained control of a southern city seized this week by a rebellious Shiite militia, the military said, while a brief halt in the U.S. assault on Fallujah fell apart as the first anniversary of the fall of Baghdad was marked by more violence.

U.S. troops fanned out across Kut, southeast of the capital, after meeting little resistance, witnesses said, in a major foray by the American military into the south, where U.S. allies have struggled to deal with the uprising by the al-Mahdi Army, led by a radical Shiite cleric.

Meanwhile, militants were holding at least six foreign hostages in unknown locations in the country. Japan's Prime Minister Junichiro Koizumi vowed not to withdraw 530 troops in the south after kidnappers threatened to burn three Japanese captives alive unless the troops leave the country.

In Fallujah, 35 miles west of Baghdad and the scene of bloody fighting with Sunni insurgents this week - Marines called a halt to offensive operations at noon, while a delegation of city leaders met with Marine commanders, said Lt. Col. Brennan Byrne, commander of the 1st Battalion 5th Marine Regiment.

But 90 minutes later, Marines were given the go-ahead to resume operations, Byrne said. U.S. forces were heard firing into the city soon after. The reasons for the end of the suspension were not immediately clear, but it appeared negotiations never took place.

The heavy siege of the city, a bastion of anti-U.S. Sunni guerrillas, has angered even pro-U.S. Iraqi officials.

"These operations were a mass punishment for the people of Fallujah," Adnan Pachachi, a senior member of the U.S.-appointed Governing Council, told Al-Arabiya TV. "It was not right to punish all the people of Fallujah and we consider these operations by the Americans unacceptable and illegal."

The top U.S. administrator in Iraq, L. Paul Bremer, said in a statement that the halt in operations also aimed to allow humanitarian supplies into the city and "allow residents of Fallujah to tend to wounded and dead."

Shooting was still heard in the city after the halt was called.

Five days of heavy fighting using tanks, warplanes and helicopter gunships in residential areas of the city of 200,000 has killed more than 280 Iraqis and at least four Marines.

Insurgents, armed with automatic weapons and rocket-propelled grenades, have put up stiff resistance, but Marines have said they are winning the battle, holding at one point around a quarter of the city.

Scores of Fallujah residents tried to leave the city during the brief pause in fighting, said Byrne. Troops used loud speakers overnight to tell people that old men, women and children would be allowed to leave, but not "military-age men."

"We told them that they would be afforded the opportunity to leave and they are leaving," he said, adding that a long line of cars was lining up to be checked by Marines before being allowed out.

The newly invigorated insurgency - Sunni rebels in the west and Shiite guerrillas in central and southern regions - further threatens shaky Iraqi security as the U.S.-led Coalition Provisional Authority prepares to hand sovereignty to an Iraqi government on June 30.

Today marked the first anniversary of the capture of Baghdad by invading U.S. forces that toppled Iraqi leader Saddam Hussein - symbolized by the April 9, 2003 toppling of a statue of Saddam by Marines with a crowd of cheering Iraqis in central Baghdad's Firdos Square.

That image became a symbol of liberation from his dictatorial regime.

But there was no celebrating today and the capital was tense. U.S. forces imposed a curfew in the morning around Firdos Square. At the western entrance to the capital, gunmen freely roamed the main highway, destroying a tanker truck today that sent a huge pall of smoke over the city.

Army Lt. Gen. Ricardo Sanchez, the top U.S. general in Iraq, vowed on Wednesday that coalition forces were launching a new operation dubbed "Resolute Sword" to destroy al-Sadr's al-Mahdi Army militia, which besides Kut was in control of the southern city of Kufa and the central part of nearby Najaf.

U.S. forces moved into Kut two days after Ukrainian forces abandoned the city in the face of heavy fighting with al-Sadr followers. Police in several cities have also abandoned their stations or stood aside as the gunmen roam the streets - raising concerns over the performance and loyalty of a force U.S. administrators are counting on to keep security in the future Iraq.

In Najaf, a policeman watched helplessly on Thursday as a pickup truck carrying a dozen heavily armed Shiite militiamen went past his police station - already in the militia's hands.

"Look, how can we control such a situation?" he asked an Associated Press reporter.

U.S. forces that swept into Kut before dawn seized police stations, forcing out both Iraqi police and militiamen and confiscating all police weapons stores throughout the city, witnesses said. There was little resistance. During the day, Americans were out in force, patrolling Kut's streets.

Coalition forces also have moved in to block the road between Kufa and Najaf, a senior aide to al-Sadr, Sheik Qays al-Khaz'ali, told The Associated Press.

Al-Sadr, a young, firebrand anti-U.S. cleric, is thought to be holed up in his office in Najaf, protected by scores of gunmen. He has said he is willing to die resisting any U.S. attempt to capture him.

He attempted Thursday to rally Iraqis - including Sunnis - behind him.

"This ordeal has shown that all the Iraqi people are united," he said in a statement issued by his office.

Sanchez said the presence of thousands of Shiite pilgrims in Najaf this weekend was hampering coalition forces from moving against militiamen who hold police stations and are in the streets around Shiite shrines in the city center.

Hundreds of thousands of pilgrims are in southern cities, particularly Karbala, ahead of al-Arbaeen ceremonies this weekend to mark the end of the period of mourning for a 7th-century martyred Shiite saint.

The kidnapping of foreigners pointed to a dangerous new tactic by militants - the use of hostages to pressure U.S. coalition allies in Iraq.

TV pictures aired in the Middle East by the Al-Jazeera satellite network and rebroadcast during prime time in Japan showed the three Japanese - two aid workers and a journalist - wide-eyed and moaning in terror as their black-clad captors held knives to their throats, shouting God is Great in Arabic.

The three were reportedly abducted in southern Iraq, but it was not clear when and by whom.

Two Arab aid workers from Jerusalem - one who had once lived in Georgia - were abducted in a separate incident, and a Syrian-born Canadian humanitarian aid worker for the International Rescue Committee was taken hostage Wednesday by a local militia in Najaf.

A representative of the security firm, Blackwater USA, also told the New York Times that four American contractors who were killed and mutilated in Fallujah last month were lured into an ambush by members of the Iraqi Civil Defense Corps. The men were working for Blackwater when their vehicle was hit by rocket-propelled grenades.

The Iraqi men had promised the Blackwater-led convoy safe passage through the city, but instead, they suddenly blocked off the road, preventing any escape from waiting gunmen, Patrick Toohey, Blackwater's vice president for government relations said in today's editions.

Two senior Pentagon officials said Thursday that they could not confirm the conclusions of the Blackwater investigation and that a separate military inquiry was continuing.