Eight men detained by Vietnam on suspicion of hijacking a Malaysian-flagged tanker speak Indonesian and were carrying large amounts of cash when they were apprehended, state media reported Sunday.

The group were taken into custody on Friday after they pulled up to Vietnam's southwestern Tho Chu island in a lifeboat claiming to have encountered an accident at sea.

The same day Malaysian authorities said eight men who had commandeered the MT Orkim Harmony a week earlier had managed to give warships the slip by escaping in a lifeboat under cover of darkness.

The vessel was the latest to be targeted by increasingly bold pirates behind an upsurge of sea hijackings in Southeast Asia in the past two years that have typically targeted smaller tankers carrying valuable petrol, diesel or gas oil.

VNExpress Sunday said the eight men -- aged from 19 to 61 -- spoke in Indonesian and "could not explain the origin of a big volume of foreign currency they were carrying and dozens of phones".

"These suspects were very stubborn, refusing to cooperate. They were professional and were very calm," Lieutenant Do Van Toan of Vietnam's Marine Police was quoted as saying.

Another marine police official, Colonel Le Van Minh, said investigators "have enough grounds to charge these eight suspects."

"Vietnam is actively cooperating with Malaysia in the case. The point is how to make them admit their crimes," he added.

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Indonesia's state news agency said Jakarta had ordered its Hanoi embassy to find out the nationality of the men.

MT Orkim Harmony, carrying around 6,000 tonnes of petrol worth an estimated $5.6 million, went missing on June 11 en-route from Malaysia's western coast to the port of Kuantan on the east coast.

The vessel's 22 crew members were unscathed except for a slightly injured Indonesian seamen who was being treated for a gunshot wound to the thigh, the Malaysian navy said Friday.

The pirates had managed to slip away by ordering naval vessels to stay at least five nautical miles from the ship or the crew would be harmed.

The London-based International Maritime Bureau has repeatedly warned that Southeast Asian waters are now the world's most piracy-prone, calling for decisive action by regional authorities to prevent the situation spiralling out of control.