The Pentagon will send 560 more troops to Iraq to help prepare for a long-awaited assault on Mosul, the Islamic State's self-declared capital in the country, possibly before President Obama leaves office.
Defense Secretary Ashton Carter announced the escalation – a boost of more than 10% in U.S. forces deployed in Iraq – during a visit Monday to Baghdad to meet with senior Iraq officials and U.S. commanders and troops.
The increase brings the number of authorized U.S. military personnel in Iraq to 4,647. But that tally doesn't include special operations forces and temporary deployments that bring the total to more than 5,000.
Carter said many of the new troops will work from an air base near Qayyarah, which was recaptured Saturday by Iraqi forces and is about 40 miles south of Mosul.
Backed by U.S.-led coalition warplanes, Iraqi ground forces and their allies have recaptured several major cities in recent months, including Tikrit, Ramadi and Fallujah, and have sought to cut off roads leading to Mosul.
"Despite the summer heat, our Iraqi partners – with your intrepid support – pressed ahead with the fight and cleared one town after another, dealing ISIL a series of blows," Carter told U.S. troops at Baghdad's airport, using an acronym for the militant group.
"With these additional U.S. forces, we'll bring unique capabilities to the campaign and provide critical support to Iraqi forces at a key moment in the fight," Carter said.
The Pentagon said the new U.S. forces will include engineers, logistics personnel and other military advisors, and that Qayyarah will "become a vital springboard" to retake Mosul.
U.S. officials said a team of American troops went into Qayyarah on Sunday for a quick site assessment and then withdrew. One likely job is helping Iraq troops build bridges to get across the Tigris River into Mosul.
The Qayyarah-based force will be allowed to accompany Iraqi troops at the battalion level, and thus closer to the front lines than U.S. troops who are largely confined to Iraqi division headquarters.
"As we pursue our next plays, we and our partners will continue to look at what more we can do to create and seize opportunities and accelerate ISIL's lasting defeat," Carter said.
The announcement ran into flak in Congress, where Rep. Mac Thornberry (R-Texas), chairman of the House Armed Services Committee, complained that the war against Islamic State "cannot be won by inches" and insisted that the White House needs to submit a supplemental budget request.
Carter's daylong visit to Iraq comes after a two-day NATO summit in Warsaw, where allies agreed to increase support for countries in the Middle East and North Africa that are battling Islamic extremism.
Carter met in Baghdad with Iraqi Prime Minister Haider Abadi and other Iraqi leaders as well as Lt. Gen. Sean MacFarland, the U.S. military commander in Iraq.
The recapture of Qayyarah followed weeks of slow progress by Iraqi ground troops seeking to approach Mosul from the south. Their advance was blunted by Islamic State's extensive use of booby traps and snipers.
But coalition forces overall have notched up significant gains in recent weeks in both Iraq and Syria. Overall, coalition-backed forces have recaptured about 40% of the territory that the extremist group once controlled.
Islamic State, in turn, has launched a series of suicide bombings in Iraq in an apparent attempt to rekindle sectarian warfare between Sunnis and Shiites, and to destabilize the central government.
At least 40 people were killed at a Shiite shrine in Balad, north of Baghdad, on Friday. On July 3, Islamic State suicide bombers killed at least 292 people in Baghdad's bustling Karada area.
The White House would like to capture Mosul and deal a decisive blow to the militants before Obama leaves office in January. Previous efforts to advance on the heavily defended city slowed or stalled, so the timing remains uncertain.
Mosul fell quickly when convoys of heavily armed Islamic State fighters stormed out of neighboring Syria and overran much of western and northern Iraq two years ago. Islamic State has held the northern city, one of Iraq's largest, since June 2014.
The stakes are high for Abadi's fragile government in Baghdad, which has struggled to quell growing public discontent. Abadi fired the head of security for Baghdad and other security officials on Friday, according to a statement from his office.
9:11 a.m.: This story has been updated with additional details.
5:51 a.m.: This article has been updated with additional background.