Russia has laid down the sixth Kilo-class submarine being built for the Vietnam Navy at Admiralty shipyards in St Petersburg.
Developed as part of a $2bn contract to deliver six submarines, the new subs are aimed at strengthening Vietnam's defences.
RIA Novosti quoted an anonymous defence industry source as saying: "This is the latest submarine in this series."
Vietnam has already taken delivery of two submarines, with the third currently undergoing sea trials and a fourth being floated out in late March. The final two are currently under construction.
"Two submarines have already been transferred to the Vietnamese Navy, [and] the third is to be transferred this year and the remaining three in 2015-2016," the source added.
Featuring stealth technology with an extended combat range to strike targets on land, sea surface and underwater, the improved Kilo-class submarines are capable of carrying out anti-shipping and anti-submarine missions in shallow waters.
Equipped with 18 torpedoes and eight surface-to-air missiles, they also feature low noise, advanced-hull architecture and an optimal level of control-process automation, while offering high reliability with minimal maintenance.
They can cruise at a speed of 20k with a range of 400 miles. In addition, they can accommodate a crew of 52 and patrol for 45 days.
Image: The Russian-built Kilo-class submarine is claimed to be one of the quietest diesel-electric vessels. Photo: courtesy of DoD.