The United States has ordered air strikes against Islamic State militants in the city of Tikrit, a senior US official has said.

The order follows a request for assistance by the authorities in Baghdad, according to the Associated Press news agency.

Earlier the Pentagon said an Iranian-backed Iraqi ground offensive in the city had "stalled".

Until now, Washington has had no involvement in the operation in Tikrit.

A reporter with AP in the city reported hearing warplanes overhead on Wednesday night, followed by multiple explosions.

However, reports of any air strikes have not been confirmed by the US.

The operation to retake Tikrit, which lies about 160km (100 miles) north of Baghdad, began earlier this month with more than 20,000 soldiers, police and Shia militiamen from the Popular Mobilisation (Hashid Shaabi) units attacking from all directions.

'Eye in the sky'

Iranian military advisers - led by Gen Qasem Soleimani, commander of the Revolutionary Guards' Quds Force - helped co-ordinate the assault.

Despite having no support from coalition aircraft, the government's forces made rapid advances, capturing outlying towns and villages along the River Tigris and entering northern and southern districts of the city.

But the offensive has since stalled in the past two weeks, with the army and militia suffering heavy casualties and the city centre remaining firmly in the control of several hundred IS militants, who have planted a large number of bombs in roads and buildings.

"Frankly [the operation] has not gone forward recently," Pentagon spokesman Col Steve Warren told reporters in Washington earlier on Tuesday.

Col Warren said Iraqi troops had encircled Tikrit, but had only made "minor inroads" on the city's outskirts.

"The enemy is dug in there," he added, with "fairly hardened and sophisticated defences".

Earlier, a senior coalition military official told the AFP news agency the US had begun providing intelligence, surveillance and reconnaissance (ISR) support on Saturday at the request of the Iraqi government. The support took the form of "an eye in the sky", the official added.

The need for coalition air support had been a point of contention between the Iraqi military and the Popular Mobilisation, which had opposed it.

Hadi al-Amiri, the head of the powerful Iranian-backed Badr Brigade militia, told journalists on Sunday: "Some of the weaklings in the army... say we need the Americans, while we say we do not need the Americans."

BBC