BEIJING (AP) — China's foreign ministry said Monday it has protested to the United States over a stopover by Taiwanese President Tsai Ing-wen during which she toured the National Aeronautics and Space Administration in Houston.

Tsai made the visit while returning from a trip to Paraguay and Belize, two of only 18 countries that extend diplomatic recognition to Taiwan, which China claims as its own territory.

Ministry spokesman Lu Kang told reporters at a daily briefing that Beijing would "always oppose any country providing conveniences and venues for relevant people from Taiwan to conduct such activities."

"We have made clear our solemn position repeatedly to relevant countries," Lu said. "We are of course against all countries that have diplomatic ties with China, including the U.S., conducting official exchanges with Taiwan."

The NASA visit marked the rare presence of a Taiwanese elected official at an official U.S. government facility.

While in transit, Tsai met with members of the Taiwanese-American community and spoke at the Ronald Reagan Presidential Library in California, the first public address by a Taiwanese leader in the United States in more than a decade.

"I want to thank everyone involved for making my #Houston stopover a wonderful one filled with good memories," Tsai tweeted. "My administration will continue strengthening every aspect of #Taiwan-#US relations. Until next time!"

Tsai's U.S. stopover comes amid deepening tensions between the U.S. and China over trade, the alleged pilfering of commercial secrets and Beijing's militarization of islands it controls in the South China Sea.

The administration of U.S. President Donald Trump will hold six days of hearings starting Monday in Washington on the next barrage in an escalating trade war between the world's two largest economies: Trump's proposed tariffs of 10 percent to 25 percent on $200 billion in Chinese goods that could kick in as early as next month.

Meanwhile, the administration has been boosting ties with Taiwan that have been technically unofficial since Washington cut them in 1979 upon recognizing Beijing.

Trump this year signed the Taiwan Travel Act encouraging high-level visits between the two sides, and the Department of Defense agreed to give American contractors marketing licenses for diesel-electric submarine technology sought by Taiwan's armed forces.

The U.S. also approved a $1.4 billion arms sale to Taiwan last year and is opening a sprawling new de facto embassy in a suburb of the capital, Taipei, in recognition of the closeness of relations.