The US will send an additional 560 troops to Iraq to help secure a newly retaken airbase as a staging hub for the long-awaited battle to recapture Mosul from the Islamic State of Iraq and the Levant (ISIL), Defense Secretary Ashton Carter has said during an unannounced visit to the country.

Most of the new troops will be devoted to the build-up of the Qayara airbase, about 64km south of Mosul, and include engineers, logistics personnel and other forces, Carter said on Monday. They will help Iraqi security forces planning to encircle and eventually retake Mosul, Iraq's second largest city.

"These additional US forces will bring unique capabilities to the campaign and provide critical enabler support to Iraqi forces at a key moment in the fight," Carter said.

He revealed US President Barack Obama's decision as he spoke to about 120 troops in a building at Baghdad's airport. The increase brings the total US force in Iraq to 4,647, and comes just three months after Obama's last troop addition.

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Congressional Republicans have long called for a more robust US deployment in Iraq. Some have even argued the US should have never left in the first place.

But Al Jazeera's State Department correspondent Rosiland Jordan said some members of Congress have warned that a sizeable deployment of US troops to Iraq would require new legal authorisation.

"Right now, [US forces] are only there on a 'train and assist' mission, and if they were to try and take a more active role in helping the Iraqi military try to launch the campaign for Mosul, there would certainly be some eyebrows raised and renewed calls for legalization for that particular activity," she said.

Carter told reporters earlier in the day that US advisers were prepared to accompany Iraqi battalions if needed, as those units begin the siege of the key northern city, but it is not clear when exactly that will happen.

American officials said a team of US troops had visited the Qayara airbase for a brief site assessment on Sunday.

Iraqi forces retook the airbase from ISIL, also known as ISIS, on Saturday. Iraqi Prime Minister Haider al-Abadi hailed the success as a key step towards retaking Mosul, saying residents there should "get ready for the liberation of their areas".

US officials said American advisers are already working at the brigade level with Iraqi special operations forces, but they have not yet accompanied them on operations.

The latest force increase came less than three months after Washington announced it would dispatch about 200 more soldiers to accompany Iraqi troops advancing towards Mosul.

In April, Obama gave the go-ahead for American troops to assist Iraqi forces at the brigade and battalion levels, putting US soldiers much closer to the battlefront - although still behind the frontlines.

The role of US troops had previously been limited to advising at headquarters and division levels, much further from any active frontlines.

Iraqi forces were already improving the base's perimeter in case of a counterattack from the nearby town of Qayara, which is still controlled by ISIL, according to US officials in Baghdad. ISIL has suffered a number of territorial losses in recent months, including the Syrian town of al-Shadadi, taken by US-backed Syrian forces in February, and the Iraqi recapture of Ramadi in December and Fallujah last month. Abadi has pledged to retake Mosul by the end of the year.