A successful bid would make it Vietnam's biggest corporate deal; Kuwait Petroleum Corp unit has bid close to $2 bln for the stake.

Petrovietnam has bid for a stake in Murphy Oil Corp's Malaysian oil and gas assets, challenging other suitors including Mitsubishi Corp and Indian state oil companies, a person with direct knowledge of the plan said.

The state oil company's bid comes as Vietnam, the third largest holder of crude oil reserves in Asia, seeks to reverse a steady decline in its oil output over the past decade. Murphy Oil and other U.S. oil companies are reducing overseas holdings as oil and gas exploration prospects improve at home.

Petrovietnam's offer is more than the $1.5 billion bid placed by the Indian consortium of Oil & Natural Gas Corp and Oil India Ltd, said the person, who declined to be identified as the sale process is confidential.

Arkansas-based Murphy Oil - which has interests in oil and gas fields in Malaysia, Vietnam, Indonesia, Brunei and Australia - is seeking buyers for a 30 percent interest in its Malaysian assets, Reuters previously reported.

Petrovietnam's "thinking is that we will be importing more oil and therefore as a state company we must be involved in oil production overseas," said Philip Andrews-Speed, an energy security specialist at Singapore's Energy Studies Institute.

The strategy is similar to that used by other state oil companies in the region facing declining domestic production, such as Thailand's PTT Pcl and Malaysia's Petronas , Andrews-Speed said.

Vietnam's previous overseas exploration forays have usually been in ex-communist countries like Russia and Venezuela, although Petrovietnam has existing joint ventures with Petronas in the Bunga Orkid and Kekwa oilfields.

Vietnam's domestic oil output has peaked as most of its new discoveries have been gas instead of oil. Vietnam is also facing trouble in some offshore exploration areas because of competing territorial claims in the South China Sea.

Last month, China moved an oil rig from disputed areas where it had been drilling for oil, diffusing some tension between the two neighbours after one of the worst breakdown in ties since they fought a brief war in 1979.

If successful, a Petrovietnam purchase of a stake in the Murphy assets in Malaysia would be Vietnam's biggest ever corporate acquisition, ahead of French firm Perenco SA's $1.29 billion purchase of ConocoPhillips' Vietnam operations in 2012, according to Thomson Reuters data.

"They are waking up to the fact other state companies in the region have left them behind," said the person with knowledge of Petrovietnam's plan.

Murphy Oil's auction of a portion of its Malaysian assets has generated interest from other bidders, including from Kuwait Foreign Petroleum Exploration Co (KUFPEC), which has offered about $2 billion for the stake, a separate source said.

Murphy is leaning towards selling the stake to passive investors - or parties who are not rival oil and gas producers - which could give Japan's Mitsubishi an edge in the sale process, said other sources with knowledge of the sale process.

Murphy Oil did not respond to an email seeking a response, sent outside of U.S. business hours.

Petrovietnam chief executive referred Reuters to its overseas development unit, whose chairman could not be reached for comment.

Petrovietnam also did not respond to an official letter seeking information on its bid for the holding in Murphy's Malaysian assets.

KUFPEC, a unit of Kuwait Petroleum Corp, did not respond to emails and phone call seeking comments.

By Denny Thomas