HANOI, March 31 (Reuters) - Vietnam's prime minister has ordered an international investigation into mysterious oil spills that have blackened some of the country's most popular beaches after ruling out the country's oil rigs.

"We have run thorough checks using vessels and aircrafts on our seas and found that our oil wells are safely operated and are not the culprit of the oil spills," Nguyen Tan Dung told delegates at the National Assembly in a live television broadcast on Saturday.

Dung was referring to two spills that occurred this year, one in January that hit beaches in the central part of the country and the second detected on March 11 along the southern coast.

State oil monopoly Petrovietnam president Dinh La Thang said tests showed the oil in the spill was crude oil from outside Vietnam, state media reported on Saturday.

Petrovietnam produces around 350,000 barrels of crude oil per day, most of it from rigs off the central coast and exported to refineries and power plants in Japan, South Korea, China, Australia and Singapore.

"The Foreign Affairs Ministry has been designated as the lead agency along with the Ministry of Environment and Natural Resources and other agencies to work with other countries in the region and in the world to find out the solution and limit the damages to our country's environment," Dung said.

The National Committee for Search and Rescue said residents had so far collected nearly 1,500 tonnes from the two mysterious spills. The oil had affected popular Danang beach and all 125 km (80 miles) of Quang Nam province's coastline, including Hoi An, a UNESCO heritage site.

Officials from Danang said on Saturday waters have been clear for several weeks now.

The second oil spill in the south has affected shrimp and mussel farms along coast, state media reported on Saturday.

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