The defence ministry on Friday confirmed it had tested a supersonic nuclear delivery vehicle in a move the United States has called an "extreme manoeuvre" amid tensions in the South China Sea. Last Sunday's launch of the hypersonic glide vehicle - which the US has dubbed the "Wu-14" - was the People Liberation Army's fourth test of the weapon in 18 months. "The scheduled scientific research and experiments in our territory is normal, and those tests are not targeted at any country and specific goals," the ministry said in response to the South China Morning Post 's query. But military observers said the frequency of the tests showed Beijing was reinforcing its nuclear deterrent in response to Washington's continued interference in China's territorial disputes in the region. US intelligence officials had described the tests as "extreme manoeuvres", according to US-based online paper The Washington Free Beacon. The latest test took place a day before Central Military Commission vice-chairman Fan Changlong left for a week-long visit to the US. Experts say the launch was timed to raise Fan's bargaining power in discussions with the US, as well as to express Beijing's disapproval of Washington's sustained interference in the South China Sea. "The test is aimed at helping Fan increase the People's Liberation Army's bargaining power on the negotiation table when he deals with his US counterpart," Macau-based military observer Antony Wong Dong said. Fan met US Defence Secretary Ash Carter in Washington on Thursday. Concerns over the disputed islands in the South China Sea were the focus of their meeting, state media reported. The Wu-14 test was probably a response to a US spy plane's flight over the South China Sea last month, said Professor He Qisong, a defence policy specialist at the Shanghai University of Political Science and Law. "The Wu-14 … is designed to penetrate US missile defence systems, meaning the PLA is capable of defending China's territorial sovereignty," He said. "But such a test is only a nuclear deterrence. Neither China nor the US wants to declare war over the South China Sea issues." Tensions have risen between the world's two largest economies, after Washington called for "an immediate and lasting halt" to China's placing of mobile artillery on its reclaimed islands and Beijing revealed plans to expand its naval power and counter US surveillance. The successful Wu-14 test - which was more complicated than the previous three - showed China was making good progress, The Washington Free Beacon quoted Chinese weapons experts as saying. The Wu-14, which carries nuclear warheads, can travel at 10 times the speed of sound.