Authorities in the Pacific island country Palau have set fire to four Vietnamese fishing vessels caught illegally operating in the territory's waters, delivering a stern warning that they will not tolerate poaching.

The 77 crew members were transferred to two other arrested fishing boats with enough fuel and provisions to get back to Vietnam.

"This message goes to the captain and crews of these vessels. Palau guarantees you will return with nothing," President Tommy Remengesau said after the boats were burned on Friday.

"Captains will be prosecuted and jailed. Boats will be burned. Nothing will be gained from poaching in Palau. From one fisherman to another, respect Palau."

The Vietnamese boats were caught in a protected area with more than eight tonnes of sea cucumbers and reef fish on board.

Since last year, 15 boats from Vietnam have been seized with more than 25 tonnes of Palau's marine species destined for the black market in Asia.

"We have a simple message for those who try to steal Palau's marine resources: we will not tolerate poachers in our ocean," Remengesau added.

"Palau is working with our military, diplomacy, and NGO partners from around the world to get tough on illegal fishers and protect our food security."

Palau hosted workshops last month involving maritime surveillance authorities from Australia, the United States, Japan and the Pacific Islands Forum Fisheries Agency, designed to develop a comprehensive marine enforcement plan.

Palau, in the western Pacific, has a fishing-reliant economy for its population of 21,000 people spread over 250 islands.