The U.S. has failed to live up to its promises to help Iraq fight Islamic State extremists, unlike the "unconditional" assistance being given by Iran, the commander of Iraq's powerful Shiite militias alleged Friday.

In a battlefield interview near Tikrit, where Iraqi forces are fighting to retake Saddam Hussein's hometown from the militants of the so-called Islamic State, commander Hadi al-Amiri criticized those who "kiss the hands of the Americans and get nothing in return."

Iraqi forces entered Tikrit for the first time Wednesday from the north and south. On Friday, they waged fierce battles to secure the northern neighborhood of Qadisiyya and lobbed mortar shells and rockets into the city center, still in the hands of IS militants. Iraqi military officials have said they expect to reach central Tikrit in two to three days.

Reuters reports Saturday that Iraqi forces and mainly Shiite militiamen have paused their offensive for a second day on Saturday as they awaited reinforcements, a military source said.

Islamic State fighters still hold about half the city and have booby-trapped buildings and laid improvised explosive devices and roadside bombs, the source in the local military command center told Reuters.

More "well-trained forces" were needed for the street-by-street battles that recapturing the whole city would require, the source said, speaking by phone from Tikrit, He did not give a timeline for their arrival.

The Iranian-backed Shiite militias have played a crucial role in regaining territory from the Sunni extremists of the Islamic State group, supporting Iraq's embattled military and police forces.

An Iraqi government official told The Associated Press that Iran has sold Baghdad nearly $10 billion in arms and hardware, mostly weapons for urban warfare like assault rifles, heavy machine-guns and rocket launchers. The official spoke on condition of anonymity because he was not authorized to brief the media.

In November, President Barack Obama authorized the deployment of up to 1,500 more U.S. troops to bolster Iraqi forces, which could more than double the total of American forces in Iraq to 3,100. The Pentagon has made a spending request to Congress of $1.6 billion, focusing on training and arming Kurdish and Iraqi forces. According to a Pentagon document prepared in November, the U.S. is looking to provide an estimated $89.3 million in weapons and equipment to each of the nine Iraqi brigades.

The U.S.-led coalition of eight countries has launched more than 2,000 airstrikes in Iraq alone since August 2014, and the U.S. is also hitting the militant group from the air in Syria. Iraqi and U.S. officials have acknowledged the role airstrikes have played in rolling back the militants, saying the air campaign was an essential component in victories at the Mosul Dam, in Amirli, and more recently, in the crucial oil refining town of Beiji.

But the U.S. is not taking part in the operation in Tikrit, with U.S. officials saying they were not asked by Iraq to participate.

Al-Amiri, the Shiite militia commander who also is head of the Badr Organization political party, said that "help from Iran is unconditional."

He warned that Iraq should not sacrifice its sovereignty for the sake of receiving weapons and assistance from the U.S., suggesting the Iraqi government is taking instructions from Washington.

"Our sovereignty is more important than U.S. weapons," he said. "We can bring weapons from any country in the world."

Separately, Grand Ayatollah Ali al-Sistani, Iraq's top Shiite cleric, urged the government to step up its support for the Shiite militias and to take care of the families of militiamen killed in battle. His remarks were relayed by his spokesman Ahmed al-Safi in the Shiite holy city of Karbala.

As many as 30,000 men are fighting the extremists in Tikrit — most of them volunteers with various Shiite militias, Iraqi officials say. U.S. Gen. Martin Dempsey said Wednesday that up to 20,000 militiamen may be involved.

Karim al-Nouri, a spokesman for the Popular Mobilization Forces, the official name of the Shiite militias, said as many as 40 Iranian advisers are also taking part.

In its march across Syria and northern and western Iraq, the Islamic State group — also known as ISIS or ISIL — has seized cities, towns and vast tracts of land. Its predominantly Sunni fighters view Shiites as apostates and have carried out a number of massacres.

On Friday, a prominent Iraqi Sunni preacher urged authorities to prevent Shiite militias from carrying out revenge attacks on Sunnis in Tikrit. In his appeal, Sheik Abdel Sattar Abdul Jabbar cited reports of Shiite militiamen burning Sunni homes in the battle.

"We ask that actions follow words to punish those who are attacking houses in Tikrit," Abdul Jabbar said during his Friday sermon in Baghdad. "We are sorry about those acting in revenge that might ignite tribal anger and add to our sectarian problems."

Abdul Jabbar said that if the government failed to stop revenge attacks by Shiite militias, Iraq would face reignited sectarian tensions, similar to those it witnessed at the height of Iraq's sectarian wars in 2006 and 2007.

Prime Minister Haider al-Abadi last week called on his forces to protect civilians and their property in recaptured areas, vowing zero tolerance for any violations. He also urged Sunnis who may have welcomed the initial onslaught or fought beside the militants to give up their support for IS.

"I call upon those who have been misled or committed a mistake to lay down arms and join their people and security forces in order to liberate their cities," al-Abadi said.

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Yacoub reported from Baghdad. Associated Press writer Vivian Salama in Baghdad contributed to this report.