Armed pirates have seized a Malaysian tanker carrying palm oil and more than 20 crew members in the Gulf of Eden off the coast of Somalia, the fourth such hijacking in a month, a global maritime watchdog said Wednesday.

Noel Choong, head of the International Maritime Bureau's piracy reporting center in Kuala Lumpur, said the center picked up a distress signal late Tuesday and immediately notified Western naval ships patrolling the area.

A warship was sent to intercept the tanker, which was heading toward Somalia territorial waters, he said.

"We have had no communication with the vessel so far. The distress call was relayed through another ship but the tanker has been confirmed seized by pirates," Choong said.

Choong said the Malaysian tanker was taking its load of palm oil from Indonesia to Europe when it was attacked, but it was unclear when the raid took place.

"This is the fourth hijacking in a month. ... We call upon the U.N. and the international community to take immediate steps to stop the worsening situation," he said.

Once the ship enters Somali waters, pirates are likely to demand ransom for the release of the vessel and the crew, he said. He declined to give further details, citing concern for the crew's safety.

Somalia has not had a functioning government since 1991 and foreign vessels are frequently seized for ransom by pirates armed with rocket-propelled grenade launchers and automatic weapons, making it difficult and expensive to deliver aid.

In June, the U.N. Security Council voted to allow international warships to enter Somali waters to combat the problem. But its 1,880-mile coastline _ the longest in Africa _ remains virtually unpoliced.

Ships have been urged to stay more than 200 nautical miles from the Somali coast, Choong said.

The Gulf of Aden, where many of the attacks take place, connects the Red Sea and the Indian Ocean, forming one of the world's busiest shipping lanes.

Twenty-four acts of piracy have been reported off Somalia in the first half of this year.