Iraqi Shiite militias accused Washington Thursday of hijacking the operation to retake Tikrit, threatening a mass pullout even as regular government forces pressed a final assault.

The US-led coalition against the Islamic State group launched its first air strikes on Tikrit late Wednesday, in what Washington and Baghdad have described as a potential game-changer in a stalled operation.

Iran had so far been the most prominent foreign partner in Baghdad's largest operation against jihadists who swept through Iraq's Sunni heartland nine months ago.

Iran-backed paramilitary organisations have done the heavy lifting in the Tikrit operation and insisted they did not need US involvement to flush out the city's last IS-held pockets.

"This operation (US-led strikes) has negative results," said Jassem al-Jazaeri, a top political official from Kataeb Hezbollah, one of the main militia groups in Iraq.

"The government might come to realise this in the coming days. We hope they will reconsider and not submit to foreign pressure," he said, speaking to his group's Etejah TV channel.

The largest contingent of fighters involved in the Tikrit operation is from the volunteer Popular Mobilisation units (Hashed Shaabi).

Many of Iraq's myriad Shiite militias fight under the command of the organisation's umbrella but also have their own agendas.

According to several of their members, the main leaders of the paramilitary groups involved in the Tikrit operation were meeting Thursday to decide whether or not to pull out.

The Pentagon said it had conditioned its strikes on a greater role for Iraqi government forces and claimed Thursday that militia groups had already "pulled back".

But it was not immediately clear from sources on the ground to what extent the paramilitary groups might have pulled back.

Akram al-Kaabi, secretary-general of the Harakat al-Nujaba militia, threatened the United States.

"The US-led international coalition is trying... to hijack victory," said Kaabi, whose outfit is a splinter from the powerful Asaib Ahl al-Haq militia.

"They agreed with the government but the government did not consult the Hashed Shaabi or commanders on the ground," he said.

- Boycott threatened -

There have been no reports of US military advisers or other forces on the ground in Tikrit but he warned nonetheless that his group were on "maximum alert" to target US troops.

He also accused Washington of engineering the creation of the Islamic State group, an accusation that is widespread among Iraq's Shiite majority but transcends the sectarian divide.

Moqtada Sadr, a young and influential cleric, voiced similar views and announced that his own Peace Brigades (Sarayat as-Salam) militia would boycott any operation in which US forces were involved.

"We were surprised by the government's decision to request the invading US forces' support in the battle against Daesh (an Arab acronym for IS), which was being defeated and forced to flee by the blows of our heroes," a statement said.

"The participation of the so-called international coalition is just to protect IS militants and hijack the victories achieved by Iraqi hands," Sadr said.

Sadr's group, which waged a deadly uprising against US forces a decade ago, has not played a frontline role in the Tikrit offensive and is perceived as having more distant relations with Iran than some other leading militias.