US-led coalition warplanes entered the Iraqi government bid to retake Tikrit late on Wednesday. Baghdad, whose ground assault stalled for two weeks, had previously insisted that it could retake Tikrit alone.

Islamic State (IS) insurgents seized the mainly Sunni city of Tikrit, 160 kilometers (100 miles) north of Baghdad, last June during a rapid advance across northern Iraq.

On Iraqi state television on Wednesday, Iraqi Prime Minister Haider al-Abadi said Iraqi forces focused on Tikrit had the backing of "friendly" countries and coalition, including arms, training and aerial support.

'Pounding' IS positions

Provincial council spokesman Hadi al-Khazraji said the combined Iraqi forces had begun "pounding" IS positions with shells, mortars and Katyusha rockets late on Wednesday evening, local time.

Abadi's government had previously refrained from asking for American help for the Tikrit offensive, which 20,000 Iraqi troops and Shiite paramilitaries began on March 2.

Pentagon sources said the air raids were directed against up to a dozen targets in Tikrit that had been selected during coalition surveillance flights in recent days.

Indirect collaboration?

For decades intense distrust has reigned between Washington and Tehran, but the developments in Iraq could point to greater collaboration, if only indirectly.

Shiite-led Iran has played a prominent role in providing artillery and deploying advisers to the Shiite militias which dominate Popular Mobilization units involved in Baghdad's bid to dislodge Sunni insurgents, including IS.

Recapturing Tikrit would give Abadi's government a hub for its ultimate goal of recapturing Mosul, the main northern city which IS seized last year.

Operation under 'Iraqi command'

The senior US commander of the international airborne coalition Lieutenant General James Terry omitted mention of the Iranian-backed militias and said the air raids were aimed at enabling "Iraqi forces under Iraqi command."

The US had previously said it was not coordinating any military actions with the Iranians.

Tikrit was the home city of executed Sunni former president Saddam Hussein.

ipj/rc (AP, Reuters, AFP)