Last year's contract on the delivery of six Kilo class diesel submarines to Vietnam, worth a total of $3.2 billion, is the largest deal in the history of Russian exports of naval equipment, a Russian magazine says.

The contract was signed in December 2009 during the visit of Vietnamese Prime Minister Nguyen Tan Dung to Russia.

"The construction cost is $2.1 billion, but the building of all necessary coastal infrastructure and the delivery of armaments and other equipment may bring the total to $3.2 bln, which makes this deal the largest in the history of Russian exports of naval equipment," the Export of Arms magazine says in an editorial published in its June issue.

Admiralty Shipyards in St. Petersburg will build the submarines with the rate of one vessel per year.

State-run arms exporter Rosoboronexport previously said Russia could sell up to 40 fourth-generation diesel-electric submarines to foreign customers by 2015.

Kilo class submarines, nicknamed "Black Holes" for their ability to avoid detection, are considered to be among the quietest diesel-electric submarines in the world.

The submarine is designed for anti-submarine warfare and anti-surface-ship warfare, and also for general reconnaissance and patrol missions.

The vessel has a displacement of 2,300 tons, a maximum depth of 350 meters (1,200 feet), a range of 6,000 miles, and a crew of 57. It is equipped with six 533-mm torpedo tubes.

At least 29 Kilo class subs have been exported to China, India, Iran, Poland, Romania and Algeria.

Export of Arms is a specialized Moscow-based technical and analytical magazine, published bimonthly by Russia's Centre for Strategic and Technological Analysis.

RIA Novosti