PUERTO PRINCESA, Philippines Gunmen believed to be Muslim rebels seized 20 people including two American missionaries in a dawn raid on a tourist resort in the southern Philippines on Sunday, officials said.

A third American and 17 Filipinos were also seized in the raid on Dos Palmas resort off Palawan Island, Reuters said.

Hours after the attack, a motorboat thought to be carrying the gunmen and their hostages was spotted by military aircraft near the maritime border with Malaysia, said national security adviser Roilo Golez.

"We are conducting pursuit operations ... we have deployed navy vessels in the area," Golez said. "They are traveling slowly apparently because of the many people on board."

Manila had told Kuala Lumpur that the gunmen might try to slip into Malaysian territory "and the Malaysian Navy is stepping up its border patrol", a military spokesman said.

Two of the kidnapped foreigners were an American couple who had been working with minority groups in the Philippines for 15 years and were on holiday at the resort, their group, new tribes mission, said in a statement.

The military described the raiders as "terrorists" while a police officer said they were believed to be Muslim separatist Abu Sayyaf rebels.

President Gloria Macapagal Arroyo issued a statement condemning the raid as a "dastardly criminal act by ruthless bandits".

Abu Sayyaf rebels sparked worldwide outrage last year when they snatched more than 40 foreigners and Filipinos from two tourist resorts in Malaysia and from Jolo Island in the southern Philippines.

Many of those victims were released after big ransoms were paid. Some were rescued by soldiers and others escaped. One Filipino resort worker kidnapped is still held.