Islamic State battle to dominate US-Iraq White House talks

Iraq's prime minister will plead for more US military support in the war against the Islamic State group during a first White House meeting with President Barack Obama on Tuesday.

Embarking on his inaugural prime ministerial trip to Washington, Haider al-Abadi said his top priority would be to secure a "marked increase" in the US-led air campaign and in the "delivery of arms."

Abadi arrives in the United States with swathes of his country - including the second city, Mosul - still under occupation by IS militants.

His armed forces, ravaged by years of war, desertion and underfunding, have parried the IS advance on Baghdad, but struggled to get on the front foot.

The IS group has lost control of 25 to 30 percent of the territory it holds in the country following the Iraqi offensive and nearly 1,900 coalition airstrikes, according to the Pentagon.

Pentagon spokesperson Colonel Steve Warren said the lost territory amounts to some 13,000 to 17,000 square kilometres.

Critics say Obama's drawdown of US troops in Iraq was a strategic blunder that allowed IS to flourish.

A recent Iraqi effort to wrest control of Tikrit stalled until the US military pounded IS positions with air strikes.

'We want to see more'

There has already been an increase in US support, "but we want to see more," said Abadi before boarding his plane bound for the American capital.

Obama has made degrading and destroying IS a priority, fearing the organisation's foothold in Iraq and Syria provides a base for terror operations, while destabilising those countries and the region. 

The United States is leading an international coalition waging a campaign of airstrikes against IS and also providing arms and training for Iraqi security forces.

Tuesday's talks at the White House are expected to focus on defining the next steps in that fight.

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

But amid skepticism that Iraqi forces are up to the task, Abadi last week announced the next steps may come in Anbar province.

Washington now says the attack on Mosul "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.

There is concern in Washington that Shiite militias fighting the IS group are controlled by the Iranian government rather than Abadi's.

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

"This is a long-term campaign," said one US official. "It is going to be a long, long, long haul, I cannot overemphasise that."