The US has begun air strikes against Islamic State militants in the Iraqi city of Tikrit, the United States military has confirmed.
The order followed a request from Iraqi Prime Minister Haider al-Abadi for assistance, after an Iranian-backed ground offensive in the city stalled.
"These strikes are intended to destroy [IS] strongholds with precision," said US Lt-Gen James Terry.
Until now, Washington has had no involvement in the operation in Tikrit.
Gen Terry, who is the commanding general of the US-led Operation Inherent Resolve, said the strikes would "minimise collateral damage to infrastructure".
The Iraqi ground offensive also resumed on Wednesday, targeting IS positions. The renewed efforts would be supported by the coalition, the US military statement said.
It said the aim was to dislodge IS from Tikrit, which is encircled by Iraqi forces, "once again placing the town under the government of Iraq control".
A reporter with the Associated Press in Tikrit reported hearing warplanes overhead on Wednesday night, followed by multiple explosions.
The operation to retake the city, 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 (al-Hashd al-Shaabi) units attacking from all directions.
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 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.
Col Steve Warren told reporters in Washington earlier on Tuesday that "the enemy" had "dug in".
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."
- The Popular Mobilisation (al-Hashd al-Shaabi), comprising dozens of Shia militias, takes a lead role in Iraqi operations against IS. Its fighters have also been accused of committing atrocities and acting with impunity
- It was formed by the Shia-led Iraqi government in June 2014 after the army collapsed in the face of an advance by IS across northern Iraq
- Iran provides funding, weapons and military advisers to the Popular Mobilisation militias and reportedly controls several of them directly
- The Popular Mobilisation is headed by Jamal Jaafar Mohammed, also known as Abu Mahdi al-Mohandis, a former Badr Organisation commander who is close to Iranian General Qasem Soleimani