President Barack Obama on Wednesday ordered the deployment of 450 more US troops to Iraq's Sunni heartland to advise and assist fragile Iraqi forces being built up to try to retake territory lost to Islamic State.

The plan to expand the 3,100-strong US contingent in Iraq and open a new operations center closer to the fighting in Anbar province marks an adjustment in strategy for Obama, who has faced mounting pressure to do more to blunt the momentum of the insurgents.

But with Obama sticking to his refusal to send troops into combat or to the front lines, the White House announcement failed to silence critics who say the limited US military role in the conflict is not enough to turn the tide of battle.

US officials hope that a strengthened American presence on the ground in Anbar will help the Iraqi military devise and carry out a counter-attack to retake the provincial capital Ramadi, which insurgents seized last month in an onslaught that further exposed the shortcomings of the Iraqi army.

The US advisers, who will be injected into the heart of one of the most hotly contested areas of the Islamic State campaign, will offer tactical advice to Iraqi officers on how to conduct their operations, the Pentagon said.

A complex challenge for the US troops, who will establish a training hub at the Taqaddum military base only about 15 miles (25 km) from Ramadi, will be their outreach to Sunni tribal fighters, many of whom do not trust the Shi'ite-led government in Baghdad.

US officials want to integrate them into the Iraqi army and reduce its reliance on Iran-backed Shi'ite militias who have also joined the fight against Islamic State.

Obama decided on the new troop deployment in response to a request from Iraqi Prime Minister Haidar al-Abadi, the White House said. The two leaders met while attending the G7 summit in Germany earlier this week.

"To improve the capabilities and effectiveness of partners on the ground, the president authorized the deployment of up to 450 additional US military personnel to train, advise and assist Iraqi Security Forces," the White House said in a statement.

Obama also ordered "the expedited delivery of essential equipment and materiel" to Iraqi forces, including Kurdish peshmerga troops and Sunni fighters operating under Iraqi command, the White House said.

It made the announcement two days after Obama said the United States did not yet have a complete strategy for training Iraqi security forces to regain land lost to Islamic State fighters, who have seized a third of Iraq over the past year in a campaign marked by mass killings and beheadings.

The fall of Ramadi last month drew harsh US criticism of the Iraqi military's retreat from the city. Defense Secretary Ash Carter said that Iraqi forces showed "no will to fight".

SEEKING TO SPEED UP FLOW OF TRAINEES

US forces have already conducted training at the al-Asad military base in western Anbar, and the new site will focus more on advising Iraqi forces on operations in what one US official described as an effort to "buck up the ranks".

The Pentagon said the first of the new troops will arrive at Taqaddum, in eastern Anbar, within a few days from forces already in the country. The base will also be used to help guide Iraqi efforts to reclaim Fallujah, a nearby city the militants have held for more than a year, US officials said.

Still, Obama's new plan stops short of some of the more assertive steps demanded by his conservative critics at home, such as putting US spotters in forward positions to call in air strikes or embedding American advisers with Iraqi forces in combat.

US House of Representatives Speaker John Boehner said Obama's plan to send additional US military personnel to train Iraqi forces was a "step in the right direction," but not a sufficient strategy to defeat Islamic State.

"It's clear that our training mission alone has not been enough," the Republican lawmaker said.

John McCain, Republican chairman of the Senate Armed Services Committee, said: "I remain deeply concerned that this new deployment is disconnected from any coherent strategy to defeat ISIL."

With the latest adjustments, Obama is deviating only slightly from his policy of relying on a bombing campaign and local forces without committing large-scale US troops. His options are hemmed in by a deep aversion to seeing America drawn back into Iraq after pulling out US forces in 2011.

Ben Rhodes, Obama's deputy national security adviser, said the president recognized the "inherent risk" of attack that the new US contingent could face in volatile Anbar and insisted that security precautions were being taken.

US officials took pains to insist that Wednesday's announcement did not amount to an overhaul of Obama's anti-Islamic State strategy, but they left open the possibility of further unspecified steps.

"The president hasn't ruled out any additional steps," Rhodes told reporters on a conference call. "He's always open to considering refinements." – Reuters