Philippine President Rodrigo Duterte said on Sunday he did not seek support from Washington to end the siege of a southern Philippines city by the so-called Maute group.
It came a day after the United States said it was providing assistance at the request of the government.
The seizure of Marawi on May 23 by hundreds of local and foreign fighters has alarmed Southeast Asian nations, which fear Daesh-linked Maute group is trying to establish a stronghold on Mindanao.
Duterte told a news conference in Cagayan de Oro City, about 100 km from the besieged Marawi, that he had "never approached America" for help.
When asked about US support to fight the Maute group in Marawi City on the island of Mindanao, Duterte said he was "not aware of that until they arrived."
But the Philippine military said on Saturday US forces were providing technical assistance without having "boots on the ground".
It confirms a statement from the US embassy in Manila which said the support had been requested by the government.
The Pentagon confirmed it was helping the Philippine military in Marawi.
It said in a statement on Saturday it was providing Philippine forces with security assistance and training in the areas of intelligence, surveillance, and reconnaissance.
It said it had an additional 300 to 500 troops in the country to support regular training and activities, without giving further details.
A US official, speaking on condition of anonymity, said support included aerial surveillance and targeting, electronic eavesdropping, communications assistance, and training.
US special forces have joined the battle in the southern Philippine city of Marawi, after 13 Philippine marines were killed in the intense urban fighting, the Philippine military said on Saturday.
The Marines were killed while rescuing trapped civilians in the conflict-torn city, a Philippine army spokesperson said.
More than 200 people have been killed in the fighting in its third week, said the spokesperson.
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As of Saturday, the number of security forces killed in the battle for Marawi stood at 58. The death toll for civilians was 20 and more than 100 had been killed overall.
At least 200 rebels are holed up in a corner of the city.
An estimated 500 to 1,000 civilians are trapped there, some being held as human shields, while others are hiding in their homes with no access to running water, electricity or food.
Use of air strikes
Air strikes have been directed at the at the central business district of the city where the insurgents are dug in, said the military.
"The use of air strike is to soften the position [of the enemy] and to take down battle positions of the enemy, particularly machine gun nests, sniper's nests, and the command in control of the enemy," said Col. Jo-ar Herrera, spokesperson for the Philippine army's 1st Infantry Division.
But it is proceeding carefully so as not to destroy mosques where some of the militants had taken up positions.
Warplanes hovered over on Saturday and black smoke could be seen rising in many parts of the city.
It is also believed there is a network of underground tunnels where the militants can take shelter from the bombing.
A US P-3 Orion surveillance plane was seen over the town on Friday.
The US has no permanent presence in the Philippines but for years has kept 50 to 100 special forces troops in the south of the country on rotational exercises.
'Our soldiers are pro-American'
President Duterte declared martial law across the entire southern island of Mindanao after ongoing clashes broke out on May 23 when the Maute group attacked Marawi.
Duterte said under martial law he has authority over the defence department.
He also said because of years of training from the United States, "our soldiers are pro-American, that I cannot deny."
Presidential spokesperson Ernesto Abella said in a statement US forces were participating directly in combat operations, which is prohibited by Philippines law.
"The fight against terrorism, however, is not only the concern of the Philippines or the United States but it is a concern of many nations around the world," he said.
"The Philippines is open to assistance from other countries if they offer it."
The Maute joined forces with Isnilon Hapilon, who was last year proclaimed by Daesh as its Southeast Asia "emir".
Military officials believe Hapilon is still in the city.
The military has said it is aiming to end the siege by Monday, the Philippines' independence day.