Beijing has warned Washington not to cross its “red line” on Taiwan after a US envoy arrived on the island as part of a delegation from Palau, one of Taipei’s 15 remaining allies. Chinese foreign ministry spokesman Zhao Lijian said on Monday that “the Chinese side resolutely opposes any form of official contacts between US and Taiwanese officials”, adding that any such contact would hurt US-China ties and affect stability in the Taiwan Strait. The warning came after Palauan President Surangel Whipps arrived in Taiwan on Sunday for a five-day trip to launch a “travel bubble” to ease coronavirus between Taipei and Koror, Palau’s biggest island. US ambassador to Palau John Hennessey-Niland was part of the delegation, becoming the first US envoy to visit Taiwan since Washington switched diplomatic recognition to Beijing from Taipei in 1979. Beijing, which considers Taiwan part of its territory that must be returned to its control by force if needed, has repeatedly warned the US against official contacts with the island. On Monday, a day after the US ambassador was reported to be visiting Taiwan, Beijing sent 10 warplanes to Taiwan’s southwest air defence identification zone to ramp up pressure on the island, according to Taiwan’s defence ministry. In Taipei, Whipps said his country needed support from both Taipei and Washington. “Having the US ambassador here with us is just a demonstration of how we work together to get this ‘sterile corridor’ or ‘travel bubble’ started,” he said, referring to US supplies of vaccines to Palau that enabled the formation of the travel bubble. “As a small country, we could easily be infiltrated and we depend on our partners to protect us and give us security. So I appreciate [Hennessy-Niland] joining us … which demonstrates [the US] friendship and commitment in protecting us and giving us security.” Observers said Hennessey-Niland’s presence was a strong signal from the US that it would respond to any effort by Beijing to bring Palau into its fold. “According to normal diplomatic practice, Ambassador John Hennessey-Niland would not be able to come to Taiwan without permission from the State Department,” said Alexander Huang Chieh-cheng, a professor of international relations and strategic studies at Tamkang University in Taipei. “[His visit] signals the US effort to assist Taiwan in securing diplomatic ties with Palau against pressure from Beijing.” Whipps told Taiwan’s semi-official Central News Agency that soon after he was elected president late last year, Beijing tried to persuade him to switch diplomatic recognition to the mainland but he rebuffed Beijing’s overtures, saying he valued Palau’s ties with Taipei. Hennessey-Niland has said Pacific nations need to be aware of the risks and the potential loss of autonomy in siding with Beijing. And in a US Senate hearing in December 2019, he said Taiwan was a US partner and critical to helping contain Beijing’s military expansion in the Indo-Pacific region. Huang said Hennessey-Niland’s visit “implies that the Biden administration has observed the Taiwan Travel Act and the decision made by its predecessor to lift unfair and unnecessary restrictions on certain official engagements with Taiwanese counterparts”. Former US president Donald Trump approved the Taiwan Travel Act in 2018 to allow high-level official visits with Taipei. His administration also removed decades-old, self-imposed restrictions on how its diplomats and other officials interact with the island. Observers said the US envoy’s presence on the trip also indicated that US partnership with Taiwan had gone beyond just the US-Taiwan level, but also involved a third party such as Palau. “It looks like a concerted action,” Huang said, adding that more evidence was needed to assess whether the three had established a formal partnership. Lin Ting-hui, deputy secretary-general of the Taiwan Society of International Law, said he did not rule out a joint coastguard drill between Taiwan, Palau and the US, given that Palau had a coastguard agreement with Taiwan and the US had a similar arrangement with Taiwan and a defence deal with Palau. According to The Wall Street Journal , Palau had asked the Pentagon to build ports, bases and airfields in the island state , offering a boost to US military expansion plans in Indo-Pacific region, as Washington aims to counter China.