HANOI (AFP) - A Vietnam-flagged tanker was hijacked by gun-toting pirates who stole part of the vessel's cargo of oil before releasing the ship and its crew safely, an official said.

The MT Sunrise 689 went missing en route from Singapore to the Vietnamese port of Quang Tri, falling out of contact shortly after it left port a week ago.

"The Sunrise and its 18 crew members were released early this morning by pirates who took around a third of cargo on board," Nguyen Nhat, the director of Vietnam's Maritime Department, said on Thursday.

"Around a dozen pirates with guns jumped on the ship, took control and beat the crew," said Nhat, who had spoken to the ship's captain Nguyen Quoc Thang early Thursday after their release.

"The pirates broke the communication system, robbed the oil and goods on board," he added.

Southeast Asia has seen a spate of daring hijackings this year, centred on the Strait of Malacca running between Malaysia, Indonesia and Singapore.

The incidents have fanned fears that the region's busy shipping lanes - plagued by piracy for centuries - could once again become a problem area after an earlier surge in sea banditry was largely suppressed by regional navies.

The Sunrise, which belongs to a shipbuilding company in the northern Vietnam port of Hai Phong, was carrying more than 5,200 tons of oil and 18 crew, state media reported.

"Two crew members were slightly injured," Nhat said, adding that the others were unhurt.

Vietnam's Ministry of Foreign Affairs spokesman Le Hai Binh said late Thursday that the MT Sunrise 689 was "now in Vietnamese waters (and) rescue vessels have been sent out to help." Most recent piracy attacks have involved tankers whose cargo of oil or fuel have been siphoned off to other vessels. There have been no reports of deadly violence.

"Vietnam condemns the (MT Sunrise 689) robbery, and calls on countries in and outside the region to cooperate closely to fight piracy... and ensure maritime security, safety and freedom," Binh said.

In recent years, global concern over piracy has focused on attacks by trigger-happy Somali pirates off East Africa.

An international naval effort has virtually stamped out that threat, but South-east Asian piracy attacks have crept back up, according to the International Maritime Bureau.

The agency in June called on regional authorities to step up cooperation to prevent a re-emergence of the piracy scourge in the Malacca Strait and adjacent waters - conduit for one-third of global trade flows.

Anti-piracy experts have said an increasing booty of oil and other cargo floating through the seaway appeared to be drawing in new players, possibly underpinned by organised criminal syndicates.