U.S. special forces have joined the battle to crush Islamist militants holed up in a southern Philippines town, officials said on Saturday, as government forces struggled to make headway and 13 marines were killed in intense urban fighting.

The Philippines military said the United States was providing technical assistance to end the siege of Marawi City by fighters allied to Islamic State, which is now in its third week, but it had no boots on the ground.

“They are not fighting. They are just providing technical support,” military spokesman Lieutenant Colonel Jo-Ar Herrera told a news conference in Marawi City.

The Pentagon, which 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, confirmed it was helping the Philippine military in Marawi.

It said in a statement 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.

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