When China flexed its military muscle in the South China Sea on Wednesday, it put the PLA’s most advanced land-based anti-ship ballistic missile to the test: the “aircraft-carrier killer”. The DF-26B was fired into the northern area of the disputed waterway from Qinghai province in China’s northwest, a source close to the military said , in a move seen as a warning to the United States. They said a DF-21D anti-ship ballistic missile was also launched from Zhejiang province in the east. However, a US defence official said the Chinese military had launched four medium-range ballistic missiles in the region on Wednesday, though they had yet to identify them, according to Reuters. The missiles were fired a day after China said a US U-2 spy plane had entered a no-fly zone without permission during a Chinese live-fire naval drill in the Bohai Sea off its north coast, and amid escalating tensions between Beijing and Washington in the region. It is not known whether the missiles hit any targets, but a US Air Force missile-tracking spy plane was dispatched to the area, flight tracking information showed, apparently to monitor and collect intelligence on the warheads. China was not a signatory to the Intermediate-Range Nuclear Forces (INF) Treaty agreed by the US and Soviet Union towards the end of the Cold War, but its DF-26 and DF-21D are types of weapons banned under the pact. When the US withdrew from the treaty last year, it cited China’s deployment of such weapons as justification. It also said Russia had violated the treaty. From the launch site in Qinghai to the target area in the South China Sea, the “aircraft-carrier killer” would have travelled about 2,500km (1,550 miles), while the distance from Zhejiang for the DF-21D was about 1,600km (995 miles). ‘If China attacks our navy, we’ll call the United States’: Philippines The two missiles have a range of 4,000km (2,485 miles) and 1,800km (1,120 miles), respectively. Travelling out of the atmosphere then re-entering at more than 20 times the speed of sound, they are much faster and harder to intercept than cruise missiles. The DF-21D is designed to strike large, moving surface ships, while the nuclear-capable DF-26 can hit both surface and ground targets. Apart from its road-mobile “aircraft-carrier killer”, unveiled in 2015, the People’s Liberation Army also has the world’s first hypersonic glide missile in service, the DF-17, which was revealed in 2019 . But the US and Soviet Union were banned from developing such land-based ballistic or cruise missiles with a range of between 500km and 5,500km (310 miles and 3,420 miles) under the INF treaty they signed in 1987. Instead, they shifted focus to air- and sea-based missiles, and intercontinental ballistic missiles. Pressure from western Europe played a big part in the treaty, because missiles in that range would not reach from Moscow to Washington, but they could pose a significant threat to European nations if the two Cold War rivals exchanged fire. Since it pulled out of the treaty, the US has resumed research and development of intermediate-range missiles and is looking for missile bases in the Indo-Pacific region, which US Secretary of Defence Mark Esper on Wednesday described as “the epicentre of a great power competition with China” . The PLA is also believed to have test-fired anti-ship ballistic missiles into the South China Sea in July last year. The US made the claim after the Chinese authorities marked out two no-fly zones there – one of which covered a similar area between Hainan Island and the Paracels, where the missiles were believed to have been fired on Wednesday. China denied the accusation , saying its military had conducted routine live-fire drills.