As concern has increased about China's aggressive claims to almost all of the disputed water, Vietnam has been spending billions of dollars developing a submarine fleet, shore-based artillery and missile systems, multirole jet fighters and fast-attack ships, most of which have being bought from Russia and India. A submarine can be seen in the middle pier at Cam Ranh Bay in Vietnam. Credit:Google Maps Vietnam was also seeking more Russian jet-fighter bombers and was in talks with European and US arms manufacturers to buy fighter and maritime patrol planes and unarmed surveillance drones, Reuters said, quoting unnamed sources. The country has also recently upgraded and expanded air defences, including obtaining early-warning surveillance radar from Israel and advanced S-300 surface-to-air missile batteries from Russia. Vietnam's military spending had outstripped its south-east Asian neighbours over the past decade, the Stockholm International Peace Research Institute said.

Carlyle Thayer, a professor from Australia's Defence Force Academy in Canberra, said when all six of Vietnam's submarines were operational they would provide a potent strike capability with Vietnam's anti-ship and land attack cruise missiles, adding greatly to the country's ability to confront an enemy in its waters. Cam Ranh Bay has been described as Vietnam's "ace up its sleeve" against China's vastly larger and better-equipped navy, air force and army. Credit:Google Maps "These weapons systems should enable Vietnam to make it extremely costly for China to conduct maritime operations within a 200 to 300-nautical-mile band of water along Vietnam's coast, from the Vietnam-China border in the north-east to around Da Nang in central Vietnam, if not further south," Professor Thayer said in a Thayer Consultancy background briefing paper. Professor Thayer, an expert on Vietnam's military and the South China Sea dispute, said Vietnam's ability to deploy stealthily would be put at risk if China permanently stationed anti-submarine warfare aircraft on Fiery Cross Reef in the Spratly Islands, where China has built a 3000-metre airstrip and some basic infrastructure. China landed a civilian plane on the strip on January 2, sparking a furious response from Vietnam, which labelled it a "serious infringement of the sovereignty of Vietnam".

Analysts said it was difficult to gauge Vietnam's actual capabilities and how well they were integrating complex new weapons systems. But Professor Thayer said when all of Vietnam's current and future arms acquisitions were taken into account, "it is evident that Vietnam has taken major steps to develop a robust capacity to resist maritime intervention by a hostile power". The diesel-electric submarines, also known as Varshavyanka-class, are designed for anti-submarine warfare, anti-shipping and anti-surface ship warfare, patrol and reconnaissance, and for the defence of naval bases and coastlines. They are considered one of the quietest submarines, can operate in the South China's Sea's shallow water and have been upgraded constantly since the 1980s. Analysts say they are more technologically advanced than other Russian-made submarines in China's fleet.

Analysts said Cam Ranh Bay was Vietnam's "ace up its sleeve" against China's vastly larger and better-equipped navy, air force and army, against which it fought a bloody war in 1979. Vietnam has signalled it will invite non-Chinese navies such as Russia, the United States and Japan to send ships and submarines to the harbour for maintenance and logistics support. The harbour, which was the US's centre of naval operations during the Vietnam War, provides ships easy access to the disputed water and the Indian Ocean through the Strait of Malacca. The International Crisis Group has warned that the South China Sea risks becoming a theatre of big-power competition in 2016, as the US challenges China's large-scale land reclamation and construction on disputed reefs, which has set Beijing on a collision course with several south-east Asian nations. A tribunal in The Hague is expected to announce its verdict in a landmark case filed by the Philippines accusing China of violating international law in the South China Sea, further raising tensions.

Malaysia, Brunei, Taiwan, Vietnam, the Philippines and China have overlapping claims to the territory. More than $US5 trillion ($7 trillion) of trade passes every year through the South China Sea, which is also believed to hold huge deposits of oil and gas. - with Reuters Follow FairfaxForeign on Twitter Follow FairfaxForeign on Facebook