Vietnam's military is steeling itself for conflict with China as it accelerates a decade-long modernisation drive, Hanoi's biggest arms buildup since the height of the Vietnam War.

The ruling Communist Party's goal is to deter its giant northern neighbor as tensions rise over the disputed South China Sea, and if that fails, to be able to defend itself on all fronts, senior officers and people close to them said.

Vietnam's strategy has moved beyond contingency planning. Key units have been placed on "high combat readiness" -- an alert posture to fend off a sudden attack -- including its elite Division 308, which guards the mountainous north.

The two countries fought a bloody border war in 1979. The likely flashpoint this time is in the South China Sea, where they have rival claims in the Spratly and Paracel archipelagos.

"We don't want to have a conflict with China and we must put faith in our policy of diplomacy," one senior Vietnamese government official, who spoke on condition of anonymity, said. "But we know we must be ready for the worst."

Most significantly, Hanoi is creating a naval deterrent largely from scratch with the purchase of six advanced Kilo-class submarines from Russia.

In recent months, the first of those submarines have started patrolling the South China Sea, Vietnamese and foreign military officials said, the first confirmation the vessels have been in the strategic waterway.

Militarily, the tensions are palpable northwest of Hanoi at the headquarters of Division 308, Vietnam's most elite military unit, where senior army officers talk repeatedly about "high combat readiness".

Perched between Vietnam's craggy northern mountains and the ancient rice paddies of the Red River Delta, 308 is Vietnam's oldest division and still effectively guards the northern approaches to Hanoi.

Reflecting deep-set official sensibilities towards offending Beijing, one senior officer, Colonel Le Van Hai, said he could not talk about China. But Vietnam was ready to repel any foreign force, he said during a rare visit by a foreign reporter.

"Combat readiness is the top priority of the division, of the Ministry of Defense and the country. We can deal with any sudden or unexpected situation we are ready," he said.

Vietnam is seeking more Russian jet fighter-bombers and is in talks with European and US arms manufacturers to buy fighter and maritime patrol planes and unarmed surveillance drones, sources said. Indeed, increases in Vietnam's military spending have outstripped its South East Asian neighbors over the last decade, according to estimates by the Stockholm International Peace Research Institute (SIPRI).

While communist parties rule both Vietnam and China and share political bonds, the two countries have a history marked by armed conflict and long periods of lingering mistrust.