The Pentagon acknowledged for the first time it has deployed US troops to Yemen since the country's collapse last year to bolster government and Arab coalition forces battling Al-Qaeda.

Spokesman Navy Captain Jeff Davis said the US military has also stepped up air strikes against fighters with Yemen-based Al-Qaeda in the Arabian Peninsula (AQAP).

A "very small number" of American military personnel has been working from a "fixed location" with Yemeni and Arab coalition forces -- especially the Emiratis -- in recent weeks around Mukalla, a port city seized by AQAP a year ago, Davis said.

"This is of great interest to us. It does not serve our interests to have a terrorist organization in charge of a port city, and so we are assisting in that," the spokesman added.

He said the troops were helping the Emiratis with "intelligence support," but declined to say if they are special operations forces.

AQAP fighters have now fled Mukalla and other coastal areas, due to the government offensive.

While the number of US personnel on the ground is limited, the United States is also offering an array of assistance to partners in Yemen, including air-to-air refueling capabilities, surveillance, planning, maritime security and medical help.

The Pentagon previously had more than 100 special operations forces advising the army in Yemen, but pulled them out early last year as the country collapsed.

The US Navy also has several ships nearby, including an amphibious assault ship called the USS Boxer and two destroyers.

AQAP took advantage of the chaos of fighting between pro-government forces and Iran-backed Huthi rebels to expand its control in southern Yemen, including the seizure of Mukalla.

The Huthis denounced the return of the US military "with their weapons in southern Yemen and Al-Anad airbase," the largest in the country.

American personnel had been deployed at the base gathering intelligence for drone strikes on Al-Qaeda until they pulled out in March last year, shortly before the Huthis overran the area.

In a statement posted online, the Huthis threatened to "fight with all our means" the US and UAE presence in southern Yemen.

Pro-government forces backed by the Saudi-led coalition recaptured Al-Anad and other southern areas from the Huthis last year, but the rebels still control large parts of the country, including the capital Sanaa.

The Pentagon announced it has carried out a string of strikes on Al-Qaeda in recent weeks, outside of Mukalla.