Taiwan expressed thanks and China was upset on Wednesday after Donald Trump signed into law legislation requiring the U.S. State Department to regularly review and update guidelines on how the United States officially interacts with Taipei.

The United States is Taiwan’s most important international backer despite the lack of formal diplomatic ties, and the issue is a constant source of irritation in Sino-U.S. relations given Beijing views the democratically-governed island as its own.

Taiwan Foreign Minister Lin Chia-lung told reporters more frequent reviews of the guidelines would allow Taiwanese officials into federal agencies for meetings, for example, though the legislation does not make explicit mention of this.

In Beijing, Chinese Foreign Ministry spokesperson Lin Jian said China firmly opposes any form of official contact between the United States and “the Taiwan region of China”.