Authorities in Southeast Asia are searching for a Vietnamese oil tanker, which has been missing for the past six days. and is suspected to have been hijacked by pirates. The tanker was reportedly carrying more than 5,000 tons of gas oil.

Sunrise 689, which was bound for Quang Tri in central Vietnam with 18 people on board, vanished from radar contact 40 minutes after it left Singapore on Oct. 2, and local maritime authorities, believe that the vessel, owned by Vietnam's Haiphong Sea Product Shipbuilding Co., may have been hijacked by pirates. The International Maritime Bureau's Piracy Reporting Center had said in July that at least six oil tankers have been hijacked in Southeast Asian waters since April.

“It looks like their communication system is off or destroyed," Noel Choong, head of the bureau's piracy reporting center, said on Wednesday, according to Reuters, adding that an attempt to use satellites to trace the Vietnamese vessel had failed.

On Tuesday, Vietnam's National Search and Rescue Committee, and its foreign ministry, sent several diplomatic notes to Singapore, Malaysia, Indonesia, Myanmar, Philippines, Thailand, Brunei and Cambodia, asking assistance in its search for Sunrise 689.

"We learnt that parties involved have started the search for the ship. So far Vietnam has yet to send any ship to the last-known location," Nguyen Vu Diep, a company official said, according to Reuters.

Colonel Ngo Ngoc Thu, vice commander and chief-of-staff of Vietnam's coastguard, told BBC: "There is no official conclusion yet, but the possibilities of technical problems or weather are very low. Our search units are trying to verify whether pirates attacked it."

Late in August, pirates attacked a Thai tanker, V.L. 14, near Malaysia and drained the vessel of 296 tons of lubrication oil. Earlier that month, pirates also attacked another vessel, MT Oriental Glory, and stole nearly 2,500 tons of oil on board.

Piracy is on the rise in Southeast Asia, the BBC reported, especially off the coasts of Singapore and Malaysia in the Strait of Malacca, which is one of the busiest shipping channels in the world.