By Nga Pham
BBC Vietnamese Service
Vietnam has ordered six of the Russian Kilo class submarines
Vietnam has signed billion-dollar contracts to buy submarines and fighter jets from Russia, Prime Minister Nguyen Tan Dung has announced in Moscow.
The deals make Vietnam one of the key clients of the Russian arms industry.
The weapons purchases come at a time when disputes over sovereignty are increasing in the South China Sea, which Vietnam calls the East Sea.
Vietnam, China and other countries have competing claims over potentially oil and gas-rich island chains there.
"Vietnam signed contracts for the purchases of submarines and planes from the Russian side," the Vietnamese prime minister said, without elaborating.
Russian agency Interfax quoted unnamed sources as saying that Hanoi was to buy six diesel-electric Kilo-class submarines worth $2bn (£1.2bn).
Vietnam is already awaiting the delivery of eight Sukhoi Su-30MK2 fighter jets from Russia in 2010. It is considering ordering 12 more, Interfax quoted another Russian source as saying.
A regional defence analyst said the rising tension in the South China Sea was "clearly a source of concern" to Hanoi.
The submarine acquisition "would increase [Vietnam's] negotiating power in the maritime disputes", Professor Carlyle Thayer of the Australian Defence Force Academy said.