Pentagon chief Ashton Carter said that Washington will deploy 560 additional troops to aid Iraq's fight to retake Mosul from jihadists, deepening US military involvement in the country.
The announcement, which will bring the total authorised number of American military personnel in Iraq to more than 4600, came two days after Baghdad said it had recaptured an airbase south of Mosul that is seen as key for the eventual battle for the city.
Iraq's second city Mosul has been under Isis (Islamic State) control since June 2014, when the jihadists overran large parts of the country, carrying out atrocities including execution-style killings, mass kidnappings and rape.
Isis also holds territory in neighbouring Syria, but has lost significant ground in both countries, and Carter wanted to highlight successes, even as the jihadists have struck back with devastating attacks in Iraq and abroad.
"I am pleased to report today that... we agreed for the United States to bolster Iraqi efforts to isolate and pressure Mosul by deploying 560 additional troops," Carter said at the Baghdad airport following meetings with the Iraqi Prime Minister and Defence Minister.
"With these additional US forces we are describing today, we will bring unique capabilities to the campaign and provide critical support to Iraqi forces at a key moment in the fight," he said.
US President Barack Obama made ending America's nearly nine-year war in Iraq a centrepiece of his presidency, but Washington has been pulled back into the country by the war against Isis.
And while most of the US forces in Iraq are in non-combat roles, others have directly battled Isis, and three American military personnel have been killed by the jihadists.
"The additional troops will provide a range of support for Iraqi security forces, including infrastructure and logistical capabilities at the airfield near Qayyarah," the Pentagon said.
Prime Minister Haider al-Abadi announced on Sunday that Iraqi forces had recaptured the Qayyarah airbase, some 60km south of Mosul, which Isis seized in June 2014. The facility had suffered some damage and Isis fighters put up only minimal resistance.
The Pentagon said the base would become a vital "springboard" for the Iraqi forces' push on Mosul.
Lieutenant-General Sean MacFarland, commander of the US-led operations against Isis, said the "preponderance" of the 560 additional troops will be based at Qayyarah, and would start being deployed "in fairly short order".
Carter met Abadi as well as Defence Minister Khalid al-Obeidi, offering condolences for recent Isis attacks and congratulations on Iraqi advances.
Isis has carried out bloody attacks against civilians as it loses ground, including a bombing in Baghdad earlier this month that killed 292 people, one of the deadliest to ever hit the country, and an attack on a Shia shrine a few days later in which 40 died.
MacFarland said such counter-attacks were to be expected in the short term.
"As the enemy loses control over (towns) ... they lose a base of operations, they lose finances, they lose the ability to plan, to create the fake documentation that they need to get around the world," he said.
As the enemy loses control over (towns) ... they lose a base of operations, they lose finances, they lose the ability to plan
Carter said he and Abadi discussed the next moves in the war against the jihadists, including Mosul and ways the United States could help beef up security into Baghdad and assist in detecting explosives being smuggled into the capital.
The ultimate goal was "the recapture of all of Iraqi territory by the Iraqi security forces, but of course Mosul is the biggest part of that", Carter said earlier.
US defence officials say the campaign's first "10 plays" have been successfully completed in the US-led counter-Isis campaign in Iraq and Syria.
These steps include the recapture of several important areas across the two countries, including Ramadi in Iraq and Al-Shadadi, a town in northeastern Syria previously considered a strategic Isis stronghold.
Carter and Obama have been criticised for the pace of the campaign, which began in the northern autumn of 2014 and started slowly, particularly in war-torn Syria, where the United States had few assets on the ground to provide targeting information.
The Pentagon has announced several measures to speed up the war, including a revised mission to train anti-IS rebels in northern Syria and extra advisers for Iraqi forces.
Coupled with coalition air support, the results have seen Isis losing roughly half its territory in Iraq and about 20 percent of its Syria claim, the Pentagon said.