U.S. aircraft launched bombing raids to support Iraqi forces fighting to recapture Tikrit from the Islamic State group on Wednesday, after Baghdad issued a request for air power.

The offensive to take back Tikrit -- the home town of executed dictator Saddam HU.S.sein -- has stalled over the past week with jihadists defending their positions with homemade bombs.

"I can confirm that the government of Iraq has requested coalition support for operations in Tikrit," Pentagon spokesman Colonel Steven Warren said.

"Operations are ongoing."

An Iraqi special forces lieutenant colonel told AFP: "International coalition forces bombed four areas in the centre of Tikrit city."

According to the Iraqi officer, the bombing began after nightfall and was continuing periodically.

Strikes hit an area around a palace compound area and near the Tikrit hospital, he said.

Over the weekend, U.S. aircraft -- including drones -- began carrying out surveillance flights to support the Tikrit operation on the ground against the IS militants.

Other countries in the U.S.-led coalition were taking part in the air strikes, officials said.

"These strikes are intended to destroy ISIL strongholds with precision, thereby saving innocent Iraqi lives while minimizing collateral damage to infrastructure," said Lieutenant General James Terry, who oversees the command in charge of the U.S. war effort.

Although the United States and its allies have conducted air strikes elsewhere in Iraq, the Baghdad government had not previoU.S.ly asked for American help for the Tikrit offensive.

Instead, long-time U.S. foe Iran has played a prominent role, providing artillery and deploying advisers to the Iraqi Shiite militias also taking part in the operation.

But the assault has become bogged down, even though the Iraqi forces far outnumber the IS militants.

U.S. officials and military officers made no secret of their view that Iraq had made a mistake in not asking for American air power from the start and relying solely on Iran's assistance.

"Now the operation to take Tikrit really begins," one U.S. defense official told AFP.

At a Pentagon briefing earlier on Wednesday, spokesman Colonel Steven Warren said the U.S.-led coalition was the most capable and "reliable" partner for Iraq.

He said the Iraqis had made confident predictions when the offensive on Tikrit was launched earlier this month, but he said "urban combat is difficult and slow" and the coalition had unmatched military power to offer.

"I think it's important that the Iraqis understand that what would be most helpful to them is a reliable partner in this fight against ISIL," he said.

"Reliable, professional, advanced military capabilities are something that very clearly and very squarely reside with the coalition."

President Barack Obama's administration has insisted it does not coordinate military operations directly with Iran and until this week the two countries have operated in separate areas in Iraq.

But the American surveillance flights and air raids in Tikrit illU.S.trate how Washington is moving towards greater collaboration with Tehran, albeit indirectly, despite the intense distrU.S.t between the two arch-foes.

The bid to retake Tikrit, which involves thoU.S.ands of Iraqi soldiers, police and forces known as Popular Mobilization units, which are dominated by Shiite militias, began on March 2.

The operation is seen as a test for the Iraqi forces, which collapsed in retreat last year against the IS group, despite billions of dollars worth of weapons and years of training by the U.S. military.

The battle for Tikrit could clear the way for a much larger offensive to eventually retake the northern city of Mosul, which U.S. and Iraqi commanders view as a pivotal fight that could alter the course of the war. (AFP)