Taiwanese President Tsai Ing-wen’s visit to NASA headquarters drew an angry rebuke from the Chinese government, which accused the United States of jeopardizing international relations by aiding “separatist forces” on the island.

“The Taiwan separatist forces should not be aided nor have any space on the international stage,” Chinese Foreign Ministry spokesman Lu Kang said during his Monday press briefing.

Tsai toured NASA headquarters on Saturday during a “stopover” visit in Houston — an unofficial visit to the United States that she makes whenever traveling to the Western Hemisphere. China opposes those stopovers as a gift to the leader of an island the mainland Communist regime regards as a breakaway province; her visit to a federal government facility, apparently for the first time ever, drew a particularly sharp complaint from a Chinese government wary of American support for Taiwan’s independence.

“With regard to the Taiwan's leader exploiting all kinds of excuses to justify her foreign trips and separatist activities, the Chinese side has made its stern position clear to relevant countries that we oppose offering convenience and avenue for such activities,” Lu said.

The repeated references to Taiwan’s “separatist” behavior is an intensification of Chinese rhetoric about the island, although it is consistent with their past statements.

“Even though NASA is not the Pentagon, the State Department or the White House, it is still a clear sign that the U.S. is increasingly comfortable enabling government-to-government contacts with Taiwan, which Beijing has resisted at all costs and believes, if it comes to fruition, is a slippery slope to diplomatic recognition of Taipei,” Derek Grossman, a senior defense analyst at RAND Corp, told the Japan Times. “I believe this would be a red line for Beijing and provide the best chance for Chinese military action against the island.

President Trump’s administration has angered China repeatedly with small gestures to Taiwan, such as the decision to send a State Department official to witness the opening of the new headquarters for the unofficial U.S. compound on the island. The Chinese replied with a veiled threat to invade the island, saying obliquely that the United States can’t provide enough military support to protect the Taiwan.

“It won't work, even if they try to bank on foreign forces to build themselves up,” the foreign ministry spokesman said in June.

Lu limited himself to diplomatic threats on Monday. “We firmly oppose any country that has diplomatic ties with China engaging in any official interaction with Taiwan,” he said.