BAGHDAD ø Iraqi military and security reinforcements failed to arrive in Najaf for a planned military showdown with the Iranian-backed Mahdi Army of Moqtada Sadr as U.S. forces continued to hold back.

U.S. officials reported defections of Shi'ite units of the Iraqi National Guard summoned to fight the Mahdi Army. They said an entire Iraqi brigade, based in Basra, as well as its commander has refused orders to fight Shi'ite insurgents in Najaf. Other National Guard units also failed to arrive in Najaf.

"There won't be any Iraqi forces and there won't be any battle," a U.S. officials said. "We're back to square one."

Moreover, Iraqi forces have been attacked on the roads leading to Najaf, which has prevented the arrival of most reinforcements, Middle East Newsline reported.

The officials said the interim government in Baghdad has withheld orders for Iraqi forces to dislodge Shi'ite loyalists from their strongholds in central Najaf. The officials said infighting within the Iraqi government as well as defections from the military and security forces have suspended plans to launch an offensive against Sadr.

Officials said the U.S. marine force had sought to eliminate Mahdi Army strongholds last week, but were stopped by Central Command. They said the Defense Department and the military feared a collapse of the Iraqi government of Prime Minister Iyad Alawi, who twice stopped a planned offensive by the marines and the Army's Fifth Cavalry Regiment.

"It's unlikely that the U.S. forces would be the ones [to dislodge the Mahdi Army]," Defense Secretary Donald Rumsfeld said. Publicly, Iraqi officials maintained that their military and security forces will attack the estimated 1,000 Mahdi Army fighters holed up in the Imam Ali mosque and the adjacent cemetery in Najaf. Defense Minister Hazim Shaalan said he expected Iraqi and U.S. forces to launch a "decisive battle" in Najaf.

Many of the Iraqi combat troops in Najaf were from the Iraqi Commando Unit, formerly the 36th Battalion of the Iraqi Civil Defense Corps, which fought Sunni insurgents in Faluja in April. The unit, composed largely of Kurds, has been regarded as the best-trained, -equipped and combat-ready Iraqi counter-insurgency force.

Officials did not say how many Iraqi units were summoned for the offensive against the Mahdi Army but reported the arrival of about 500 troops over the last few days.

"The coming hours will be decisive and we will teach them a lesson they will never forget," Shaalan said on Wednesday. "In the coming hours they must surrender."

Shaalan said Iraq and the United States have reached agreement on what he termed an imminent battle for Najaf. He said Iraqi forces would carry out the ground offensive with the United States providing air support.

"We are in the process of completing all our military preparations," Shaalan said. "It will be Iraqis who enter the shrine. There will be no American role in this, except providing air protection and protecting some roads leading to the shrine."

Heavy fighting was reported in Najaf on Wednesday and a U.S. military official said 50 people were killed. But officials said most of the fighting was conducted by U.S. troops, with little input from Iraqi forces. They said U.S. troops were ordered not to advance on Mahdi Army positions.

"Sadr knows that we can't advance and take him out," a U.S. military source said. "He also knows that the Iraqi forces are unwilling, let alone incapable, of doing this. So, the truth is he has won. The rest is just posturing."

On Thursday, the Defense Ministry offered an amnesty to Mahdi Army insurgents. The ministry said Shi'ite fighters who surrendered their weapons and agreed to leave the mosque peacefully would not be arrested or prosecuted.

Later, State Minister Kassem Daoud told a news conference that Sadr must surrender all of his weapons, release all detainees and sign a pledge to renounce violence. Daoud said Sadr must then issue a pledge to disband the Mahdi Army.

"The military action has become imminent," Daoud said. "If these conditions are not met, then the military solution will prevail. For five days, we have been preparing for a military offensive to end this crisis."

For his part, Sadr, believed to be financed and armed by Iran, has issued conflicting statements. On Wednesday, Sadr called for a ceasefire and expressed willingness to order his troops out of Najaf. But hours later Sadr vowed he would not leave his mosque stronghold.

"Nobody is taking Sadr at his word," U.S. National Security Advisor Condoleezza Rice said. "He's someone from whom you have to see action. He said lots of words before. He has never followed through on them. And I don't think you're going to see an Iraqi government that's going to take his word."