Vietnamese coastguards and the owners of an oil tanker that went missing after leaving Singapore say they firmly believe it has been seized by pirates near Indonesia.
Communications equipment on board the Sunrise-689 appeared to have been switched off or destroyed after the Vietnamese ship disappeared.
This means the tanker cannot be detected using satellite equipment.
Vietnamese authorities have asked counterparts across SE Asia for help.
Colonel Ngo Ngoc Thu, vice commander of Vietnam's coastguard, told BBC Vietnamese that a "hijack is very likely".
He ruled out bad weather or technical faults as causes for the tanker's disappearance.
Dao Van Quang, chairman and chief executive of Hai Phong Sea Product Shipbuilding company, the tanker's registered owner, told local newspaper VN Express that no oil had been discovered at sea, meaning "it is unlikely that the ship was sunk".
He added that the Sunrise-689 was sailing to the islands between Singapore and Indonesia, where Indonesian pirates were known to operate.
"It seems the pirates had planned very carefully and restrained the crew as soon as they got on the ship," Mr Quang said. "So the crew did not have the chance to alarm us."
The Vietnamese government has not commented on the piracy theory, with the ministry of transport saying it had yet to draw a conclusion.
The Sunrise-68 disappeared last Thursday, 40 minutes into its journey to Vietnam, sparking a major search-and-rescue operation. It was last detected 115 nautical miles (213 km) north-east of Singapore.
It had been scheduled to reach Cua Viet port in Vietnam's Quang Tri province on Sunday.
Families of the missing crew members have gathered in Hai Phong, Vietnam, to wait for further news.
The International Maritime Bureau said 11 tankers had been hijacked in South-East Asia since April this year.
The bureau is currently investigating the ship's disappearance and has alerted Interpol.
In previous cases, pirates have ambushed tankers carrying oil products. The ships are taken to an unknown location and released once their cargo has been stolen.
Recently, global efforts have focused on eradicating piracy in Somalia, allowing attacks to mount in South-East Asian waters, home to a third of global trade flows.
International piracy is estimated to cost the shipping industry as a much as $8bn (£5bn) a year.