The United States’ national security adviser has warned China not to use military force against Taiwan, a day after an unannounced visit to the democratic island by a two-star Navy admiral overseeing US military intelligence in the Asia-Pacific. Robert O’Brien cautioned Beijing while touring the Philippines and Vietnam, where he spoke to his counterparts about tackling China’s growing assertiveness in the South China Sea, a region of vital shipping routes where it has made multiple disputed territorial claims. The Chinese Communist Party also claims Taiwan, a democracy of 24 million, even though it has never ruled there. It has threatened to annex the island, and in recent months has stepped up military intimidation through air incursions and invasion simulations. “We have very close ties with Taiwan. The president has made it very clear that any attempt to cause Taiwan to unify with China with anything other than persuasion or democratic election, is something that would be extraordinarily consequential for the People’s Republic of China,” said Mr O’Brien. “I can’t imagine anything that would cause a greater backlash against China around the entire world if they attempted to use military force to coerce Taiwan,” he told reporters. The US does not have formal diplomatic ties with Taipei but it views Taiwan as a democratic ally in the strategic Indo-Pacific region. It is Taiwan’s biggest arms supplier. The Trump administration has beefed up support for Taiwan in recent months - approving several big weapons sales and dispatching senior officials, including Alex Azar, the US health secretary. This weekend Reuters reported that Rear Admiral Michael Studeman, the director of the J2, which oversees intelligence at the US military’s Indo-Pacific Command, had made a secretive visit. He is believed to be one of the most high-ranking US officers known to have visited Taipei in recent years. The increased US backing for Taiwan has riled China amid rising tensions between Washington and Beijing over the South China Sea, the crackdown on the pro-democracy movement in Hong Kong and an ongoing trade dispute. On Monday, the Chinese foreign ministry hit back with a warning of its own, demanding the US “immediately stop” all official exchanges and military contacts with Taiwan and signalling that China would make “legitimate and necessary responses” without giving further details. “China firmly opposes official exchanges and military contacts between the US and Taiwan in any form. This position has been consistent and clear. We urge the US side to fully recognise the high sensitivity of the Taiwan issue,” said spokesman Zhao Lijian.