A Vietnamese oil tanker which went missing last week was seized by pirates who stole some of its cargo before releasing its crew, say officials.

The MT Sunrise 689, heading to Vietnam's Quang Tri port, disappeared shortly after leaving Singapore.

The crew said about a dozen men armed with guns and knives had boarded the ship and beaten them.

They siphoned off the oil before leaving the ship and freeing the crew on Thursday morning.

The ship, which has been carrying for more 5,000 tonnes of oil, is now making its way to Vietnam.

"The pirates broke the communication system, robbed the oil and goods on board," Nguyen Nhat, director of Vietnam's Maritime Department, told AFP news agency.

He said the pirates had taken about a third of the cargo.

The ship's deputy captain Pham Van Hoang said he believed the pirates were Indonesian.

"They put knives on our throats and threatened to kill us if we resist," he told the Associated Press by phone from the ship. One crew member broke his leg as he tried to run away, he said.

The Mt Sunrise 689 is the 12th tanker to be hijacked in South East Asia since April this year.

The International Maritime Bureau says there has been a steady rise in piracy across southeast Asia, particularly in the busy Malacca Strait between Singapore, Indonesia and Malaysia, despite regional efforts to tackle the problem.

Globally, piracy is estimated to cost the shipping industry as a much as $8bn (£5bn) a year.