Washington (AFP) - President Barack Obama will huddle with Iraq's prime minister at the White House on Tuesday to plot crucial next steps in what US officials admit will be a long fight to defeat the Islamic State.

Obama will host Haider al-Abadi and try to forge a strategy to snatch back swaths of territory lost to jihadist fighters, after Iraqi forces' tough victory in Tikrit.

Even with the help of Iranian-backed Shiite militia, Iraq's crisis-ravaged military was unable to wrest control of the city without substantial US airstrikes to soften the ground.

A senior US official described the offensive in turn as "very difficult," and "a bit of a rollercoaster."

It was also a reality check for those who hoped that Baghdad was poised to seize back territory relinquished in northern and central Iraq last year.

Tikrit had been seen as a dress rehearsal for Abadi's much-touted "Spring offensive" to dislodge jihadist fighters from the vital oil hub of Mosul.

Washington now says that attack "needs to happen when it's ready to happen" and "shouldn't be on a fixed timetable but rather when all the ducks are in a row."

"It's going to take a lot of capacity, which is going to take time to build," said one official.

- Hope in Abadi -

Obama has deep political as well as security interests in making sure Iraqi forces succeed.

Critics say his drawdown of US troops in Iraq was a strategic blunder that allowed the Islamic State to flourish.

Washington is now putting much hope in Abadi and his willingness to engage Iraq's regional, tribal and factional leaders in the fight.

"The Abadi government is different in nearly every fundamental respect from the previous government," said one official.

"It is important to recall just how serious that crisis was" when Nouri Al-Maliki was in power, he said.

"The future of Baghdad was very much in doubt," as Islamic State fighters closed in on the capital, the official added.

But Abadi's regional outreach has created some uneasy bedfellows for Washington.

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In Tikrit, "when the operation unfolded, it was heavily influenced by Shia militia volunteers," said another senior US administration official.

Their chain of command sometimes ran through Tehran rather than Baghdad.

Increasingly, the White House suggests that the next step should come in Sunni-dominated Anbar, rather than Mosul.

That could preclude deep involvement by the Shiite militia -- for fear of stoking sectarian conflict. And if the militias sit out the next round it may also stall Iran's rising influence.

Whatever the strategy, victory and stability are unlikely to come quickly.

After 1,800 US coalition airstrikes, just 25 percent of territory lost to the Islamic State last year has been regained.

The use of US ground forces has not been ruled out.

"This is a long-term campaign," said one official.

"It is going to be a long, long, long haul, I cannot overemphasize that."