Filipino security officials said they were unaware of the presence of the US ship. Nothing on its bow identified what class the vessel was, except a US flag hoisted on the deck.

Local fishermen watched in awe as the ship appeared on the horizon off Basilan Island and sailed past Zamboanga City around 8 a.m.

It was not known if the ship was involved in antiterrorism operations in Mindanao. But the US is helping the Philippine military fight terrorism in the region, where two Jema'ah Islamiyah bomb makers, Dulmatin and Umar Patek, who masterminded the 2002 Bali bombings, are believed to be hiding.

Washington offered a bounty of $11 million for the capture of Dulmatin, a Malaysian electronic expert, and Umar Patek, an Indonesian, who helped assemble the bombs that were used in the Bali attacks that killed more than 200 people, mostly holiday-makers.

Last week a group of US and Filipino soldiers held a medical mission in the town of Barira in the southern Muslim province of Maguindanao.

Some 1,000 people were given free medical services under the joint Medical Civic Action Program, which is part of the Project Bayanihan that enabled US and Filipino soldiers to work together to improve the well being of Filipino communities.

The US Embassy in Manila said that only two US warships could be near Zamboanga or Basilan but they are present in international waters.

Jacki Lyons, an embassy and JUSMAG information officer, said one of them is the USS Stockholm, which belongs to the US Seventh Fleet, but she did not identify what kind of ship it was.

The US 11th Fleet website, however, did not list a USS Stockholm as among its ships.

The other ship, she said, is a smaller, high-speed vessel (HSV) used to transport personnel and equipment. Lyons did not identify the HSV.

The HSV, Lyons said, is scheduled for a routine port visit in a week or so in Basilan or Zamboanga to load equipment.

"I don’t know of other possibilities [other than the two ships]," she said.

Lyons stressed there is no way the two ships were going to dock in a Philippine port or even enter Philippine territory right now, because Manila has not given any clearance for any US warship to do so.

"The ships would not enter the Philippine territory without permission from the government," Lyons said.

She said it is doubtful the Stockholm would dock in Basilan or Zamboanga because it was too big for either port.

The Department of Foreign Affairs spokesman, Gilberto Asuque, said he talked with US Embassy officials who made no mention about a US warship in Mindanao.

Under the Visiting Forces Agreement, Asuque said the embassy is also supposed to make an announcement if a warship will enter the country, including the posting of the vessel’s picture.