By Manuel Mogato

MANILA, June 10 (Reuters) - U.S. special forces are helping the Armed Forces of the Philippines (AFP) to end a siege of the southern town of Marawi by militants allied to Islamic State, a U.S. embassy spokesperson in Manila told Reuters on Saturday.

The seizure of Marawi by hundreds of fighters who have sworn allegiance to Islamic State, including dozens from neighbouring countries and the Middle East, has fuelled concern that the ultra-radical group is gaining a foothold in Southeast Asia.

"At the request of the government of the Philippines, U.S. special operations forces are assisting the AFP with ongoing operations in Marawi that help AFP commanders on the ground in their fight against Maute and ASG militants," the spokesperson said. ASG stands for the Abu Sayyaf militant group.

Until now there had been no confirmation that the Philippines had sought U.S. assistance in the battle for Marawi City on the island of Mindanao, which is in its third week.

The spokesperson gave no details of the U.S. involvement. A U.S. P3 Orion surveillance plane was seen flying over the town on Friday, but there has been no evidence that the United States has put troops on the ground there.

The assistance comes after months of strain between the two long-time allies that was stoked by Philippines President Rodrigo Duterte's hostility towards Washington and his pledges to throw U.S. troops out of the country.

Washington deployed special forces soldiers to Mindanao in 2002 to train and advise Philippine units fighting Abu Sayyaf militants in a programme that once involved 1,200 Americans.

It was discontinued in 2015 but a small presence remained for logistics and technical support.

The United States and the Philippines have been allies for decades. Their relationship provided Washington with a strategic foothold in Asia, and offered Manila a shield against China's assertiveness in the region.

But Duterte has openly scorned the alliance, seeing it as an obstacle to a rapprochement with China, and has repeatedly lambasted Washington for treating his country as a lackey. (Reporting by Manuel Mogato; Writing by John Chalmers; Editing by Clarence Fernandez)