US aircraft have launched bombing raids to support Iraqi forces fighting to recapture Tikrit from the Islamic State (IS) group, after Baghdad issued a request for air power, US officials say.

The offensive to take back Tikrit — executed dictator Saddam Hussein's home town — from IS jihadists has stalled over the past week.

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

"Operations are ongoing."

Other countries in the US-led coalition were expected to take part in the air strikes, officials said.

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

Instead, Shiite-led Iran has played a prominent role, providing artillery and deploying advisers to 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.

Over the weekend, US aircraft — including drones — began carrying out surveillance flights to support the Tikrit operation.

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 illustrate how Washington is moving towards greater collaboration with Tehran, albeit indirectly, despite the intense distrust between the two arch-foes.

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

AFP