An aide to radical cleric Muqtada al-Sadr appealed for talks to resolve the standoff in the holy city of Najaf, which the Iraqi interim government has warned may end violently and very soon.
"We are ready to negotiate to end this crisis and the suffering of our persecuted people ... but this government doesn't want negotiations," said Sheik Ali Smeisim, a senior al-Sadr aide.
The government has repeatedly rejected talk. As U.S. and Iraqi forces battled, Iraq's government warned earlier Tuesday that al-Sadr's fighters "they have hours to surrender" in their weeks-long standoff inside the Imam Ali Shrine or face attack.
"The decisive hours are near," said Iraqi Defense Minister Hazem Shaalan.
It was the toughest warning yet from the Iraqi government to Shiite rebels, reports CBS News Correspondent Elaine Cobbe: Throw down your weapons or be wiped out tonight.
In the southern city of Basra, Shiite militiamen took to the streets to support the Najaf insurgents.
Meanwhile, in Baghdad, assassins bombed the convoys of two Iraqi Cabinet ministers in separate attacks that killed five bodyguards but left the officials unharmed. The al Qaeda-linked militant Abu Musab al-Zarqawi's Tawhid and Jihad group claimed responsibility for the suicide attack on Environment Minister Miskhat Moumin in a statement posted on an Islamic Web site.
In other recent developments:
In his threat to the militants at Najaf, the defense minister said guardsmen would use loudspeakers to urge the followers of firebrand cleric Muqtada al-Sadr to evacuate the shrine and surrender.
Addressing Iraqi National Guard troops in Najaf, Shaalan said Iraqi troops were working to cordon off the Imam Ali shrine.
"When your brothers approach the holy shrine compound, they will direct calls of mercy to those (militants) to surrender," he said. "They have hours to surrender."
Speaking of al-Sadr, Shaalan added that if the cleric surrendered, "he will be safe and will be highly respected. But if he continues to resist, then there will be no options for him other than death or prison."
Asked earlier if the operation could happen Tuesday, he responded: "We will give the humanitarian option a greater chance ... but if we run out of patience, then this will mean that the hour for the military plan has come."
U.S. helicopter gunships and Bradley fighting vehicles were attacking militant positions in downtown Najaf, and militants returned fire with mortars and rocket-propelled grenades.
Witnesses said Iraqi forces accompanied U.S. troops into the Old City for the first time in recent days and were stationed about 200 yards from the shrine.
Iraqi officials have said that any raid on the shrine would be conducted by Iraqi forces. The presence of U.S. troops at the holy site would infuriate the nation's Shiite majority.
Both assassination attempts against the government ministers took place in Baghdad, as their convoys passed through the capital.
A car bomb exploded in the southern neighborhood of Qadisiyah as Moumin, the environment minister, was passing, ministry spokeswoman Dalal Ali said. Four bodyguards were killed, said police, and hospital officials said at least two people were injured. Interior Minister spokesman Col. Adnan Abdul-Rahman said a suicide bomber was also killed.
The second attack, a roadside bomb targeting Education Minister Sami Mudhafar on his way to work, happened about the same time in the western Baghdad district of al-Khadra. The blast killed one bodyguard and wounded two others, police said. A surviving bodyguard said the minister was not injured.
Late Monday, U.S. warplanes bombed the area of Najaf's Old City, and fires lit up the night sky, witnesses said. Ahmed al-Shaibany, an aide to al-Sadr, said shrapnel from the attack hit the shrine's golden dome, one of its minarets and the compound's outer wall.
The U.S. military denied damaging the shrine; it said an aircrew saw militants in the compound fire a rocket that clipped one of the walls and exploded 10 yards outside.
"We are not doing anything that could have caused damage to the shrine," Marine Capt. Carrie Batson said.
There was no independent confirmation of damage to the shrine, but violence earlier Monday ripped a chunk out of the outer wall of the compound.
Explosions throughout the day on Monday shook Najaf's Old City, a mix of streets and narrow, maze-like alleys at the heart of much of the fighting. Militant medical officials said at least two insurgents were killed and four wounded.
Al-Hakim Hospital said two civilians were killed and two others injured, but more casualties were reported in the Old City and could not be reached by emergency workers, said hospital employee Hussein Hadi on Monday.
Al-Sadr, whose Mahdi Army fighters are behind the uprising, has not been seen in public for many days, and police drove around Najaf with loudspeakers declaring he had fled toward Sulaymaniyah in northern Iraq. Al-Sadr's aides denied that.
"Muqtada al-Sadr is still in Najaf and is still supervising the operations," Sheik Aws al-Khafaji, the head of al-Sadr's office in the southern city of Nasiriyah, told Al-Jazeera television.