Malaysia's navy has located a tanker that disappeared a week ago in the South China Sea and was urging its hijackers to surrender, the country's naval chief said on Thursday.

The MT Orkim Harmony, which had 6,000 tonnes of petrol in its hold and 22 crew aboard, has been given fresh touches of paint and had its name altered to "Kim Harmon", according to photos released by Malaysia's navy.

A patrol vessel was now shadowing the tanker and communicating with its hijackers in a bid to secure their surrender, Royal Malaysian Navy chief Admiral Abdul Aziz Jaafar said on his Twitter feed.

"At least eight perpetrators are onboard. They are armed with pistols and parangs (machetes). They speak with Indonesian accent(s)," he added.

"All crew are safe and unharmed."

The vessel was in Vietnamese waters, about 200 nautical miles (370 kilometres) northeast of the Malaysian city of Kota Bahru, officials said.

The Malaysian-registered tanker is the latest victim of increasingly brazen pirates behind an upsurge in hijackings in Southeast Asia in the past two years.

The typical targets are usually tankers carrying valuable petrol, diesel or gas oil.

Officials have estimated the value of the MT Orkim Harmony's cargo at 21 million ringgit ($5.6 million). Its crew includes 16 Malaysians, five Indonesians and a Myanmar national.

It was en route from Malacca on Malaysia's west coast to the port of Kuantan on the eastern coast. Its owners lost contact with the ship last Thursday while it was off the southern state of Johor.

The vessel was spotted Wednesday by a search-and-rescue operation, officials said.

- Piracy hotspot -

The London-based International Maritime Bureau (IMB) has repeatedly warned that the waters of Southeast Asia were becoming the world's piracy hotspot and called for decisive regional action to thwart attacks.

Pirates are preying on slow-moving small coastal tankers, with one attack occurring every two weeks, the IMB said recently.

Pirates usually syphon off cargoes to other vessels before later releasing the tankers and crews.

The MT Orkim Harmony's owners, Malaysia's Orkim Ship Management, has said the tanker's cargo appeared untouched after analysing photos of the hijacked vessel, said Ahmad Puzi, a top Malaysian coast guard official.

Asked whether authorities might storm the vessel, he told reporters in Malaysia's administrative capital of Putrajaya, "our options are open," but he later suggested the navy preferred to take the hijackers alive.

"We want to know their network, we want to know who is their kingpin and who they are," he said.

Southeast Asia saw 38 pirate attacks during January-March, or 70 percent of the global total of 54, the IMB said in an April report, calling the frequency of regional incidents "an increasing cause for concern."

A scourge for centuries, piracy in Southeast Asian had been significantly reduced over the past decade thanks to stepped-up regional cooperation and maritime patrols, but has re-emerged.

Much of the world's trade passes through the region's shipping lanes such as the South China Sea and the Malacca Strait between Malaysia and Indonesia.

The IMB said last December that pirates shot dead a crew member on a Vietnamese tanker off the eastern coast of Malaysia, but most attacks end with no reports of casualties.