Chinese officials warned Monday that relations with the U.S. may fall apart if President-elect Donald Trump does not respect China’s “core interests” going forward. The comments were made in response to Trump’s now-infamous phone call with Taiwan’s president. “I want to stress that the Taiwan issue concerns China’s sovereignty and territorial integrity, and involves China’s core interests,” foreign ministry spokesman Geng Shuang said during a news briefing. “Upholding the ‘One China’ principle is the political basis for developing China-U.S. ties. If this basis is interfered with or damaged, then the healthy development of China-U.S. relations and bilateral cooperation in important areas is out of the question.” The warning comes hours after reports emerged indicating China flew a nuclear-capable bomber over a disputed part of the South China Sea, in a move U.S. officials say was meant to send a message to Trump. The Xian H-6 bomber, at times accompanied by fighter jets, flew along the demarcation line between China and disputed territories, including Taiwan, Fox News reported, citing U.S. officials. Pentagon officials reportedly found out about the nuclear bomber’s flight Friday, though it was not immediately clear when the flight took place. Officials have warned the flight was the first of its kind in more than 18 months.