A Russian-made submarine arrived in Vietnam's Cam Ranh Port Tuesday, the fourth of six Kil0-class submarines that Russia is delivering as part of a $2.6 billion deal the two countries signed in 2009, Thanh Nien News reported. The submarines are part of a Vietnamese effort to deter China's military amid an ongoing dispute over the South China Sea, a maritime expanse considered rich in resources that is highly valuable for commercial shipping.

In addition to the six Kilo-class submarines from Russia, Vietnam has bought accompanying Russian-made missiles, of the Klub variety, that can attack land and could potentially reach coastal cities in China, Reuters reported in April. The missiles could also attack Chinese ships and submarines. "They've given themselves a much more powerful deterrent that complicates China's strategic calculations," Carl Thayer, an expert on the Vietnamese military at the Australian Defence Force Academy, told Reuters. 

Tensions between the countries that lay claim to swathes of the sea have grown in recent years. In particular, the dispute between China and Vietnam rose sharply when China launched drilling operations at an oil rig 120 nautical miles off the cost of Vietnam in May 2014. China has also exacerbated tensions in its relationship with the Philippines by building islands on reefs surrounding the Spratly Islands, which both countries -- among others -- claim as their own.

Before Vietnam made the 2009 deal with Russia, it had little more than a few missiles and some other weapons with limited capabilities. Under the 2009 agreement, the last of the six submarines is scheduled to be delivered in 2016. Vietnam has also reportedly bought 50 of the Klub missiles, 28 of which have already been delivered. The missiles weighs two tons each and are fired from a submarine through a torpedo tube.

The most recent submarine to reach Vietnam began its journey on May 14 and was shipped by way of a Dutch cargo ship, the Rolldock Storm. Russia is also said to be training crews to run the submarines and supplying additional parts as needed.