"Let us continue showing our strong support to our courageous and heroic men and women in uniform fighting for Marawi and defending the Republic", he said.
Foreign fighters could have already been in Mindanao before fighting broke out in Marawi on May 23, and were ready to reinforce the Maute group terrorists when they rampaged through the city, Padilla said.
The cooperation between the longtime allies is significant because President Rodrigo Duterte, who came to power a year ago, has taken a hostile stance toward Washington and has vowed to eject United States military trainers and advisers from his country. The local troops are having a hard time ending the three-week siege of Marawi City, where an estimated 200 militants have taken positions in a corner of the town where an estimated 500 to 1,000 civilians are trapped.
"At the request of the government of the Philippines, US special operations forces are assisting the (Armed Forces of the Philippines) with ongoing operations in Marawi", the embassy said, according to the Philippine news service Rappler.
The BBC's Jonathan Head says there are several reports that the Maute brothers, who lead the Maute group, are among the dead, with intercepted communications from jihadist groups suggesting this.
Philippines correspondent David Santos, who is now less than five kilometers from the fighting, said the progress of government forces has been slowed by concerns over civilians caught in the crossfire.
At least 20 civilians and around 138 militants have also been killed, the government said.
The American forces were stationed per the Philippines' request to help defeat Abu Sayyaf, military officials added; ISIS previous year pronounced the leader of Abu Sayyaf the "emir" of Southeast Asia, according to Reuters.
Reports coming in at AFP headquarters in Camp Aguinaldo, Quezon City indicated that a major engagement, lasting for some 14 hours, erupted between Marine units and Maute Group terrorists at Barangay Lilot Madaya in Marawi City Friday.
The Philippine military said that it is confiend that the war will end by 12 June, which is the country's Independence day.
A US P-3 Orion surveillance plane was seen over the town on Friday.
In defiance of the terrorists who continue to hold their ground, government troops in Marawi on Monday raised the Philippine flag in key areas in the city. "The Philippines is open to assistance from other countries if they offer it", Abella said in a statement. ISIS is an abbreviation for the Islamic State group. The news that U.S. special forces were assisting Philippine soldiers broke out when a US-P3 Orion was hovering the city on Saturday and it caught the attention of many residents.
The assistance comes after months of strain between the two longtime allies that was stoked by President Duterte's hostility toward Washington and his pledges to throw USA troops out of the country.