Taiwan has sent ships and aircraft to shadow a Chinese aircraft carrier group through the narrow Taiwan Strait, its defence ministry said on Wednesday after Chinese President Xi Jinping offered his strongest warning against Taiwan separatism to date. China claims Taiwan as its sacred territory and considers the self-ruled island a wayward province, which Xi said on Tuesday would face the “punishment of history” for any attempt at separatism.

Taiwan’s defence ministry said the carrier group, led by China’s sole operational aircraft carrier, the Liaoning, entered the waterway late on Tuesday but kept on its western side. By midday on Wednesday, it had left Taiwan’s air defence identification zone heading southwest, the ministry said, adding it looked like China was conducting drills. Taiwan’s military sent ships and aircraft to shadow the carrier group the entire way, but spotted nothing out of the ordinary and people in Taiwan should not be concerned, it added. China’s Defence Ministry did not immediately respond to a request for comment.

In January, the Liaoning sailed twice through the Taiwan Strait in what China said was part of routine drills. Taiwan says China has ramped up military exercises around the island in the past year or so. The island is one of China’s most sensitive issues and a potential military flashpoint. China’s hostility towards Taiwan has risen since the 2016 election of President Tsai Ing-wen, a member of the island’s pro-independence Democratic Progressive Party.

China suspects Tsai wants to push for formal independence, which would cross a red line for Communist Party leaders in Beijing, though Tsai has said she wants to maintain the status quo and is committed to ensuring peace. China’s Taiwan Affairs Office on Monday expressed anger at Taiwan Premier William Lai’s description of the island as a sovereign independent country, calling it a “serious provocation” and denying that Taiwan was, or could ever be, independent.

China has also been infuriated by a law signed last week by US President Donald Trump that encourages the United States to send senior officials to Taiwan to meet Taiwanese counterparts, and vice versa. US Deputy Assistant Secretary of State Alex Wong is in Taiwan this week, where he is set to speak at a business event in Taipei later on Wednesday with Tsai.

Separately on Wednesday, China announced that a former ambassador to the United Nations, Liu Jieyi, has been appointed the head of China’s Taiwan Affairs Office. Veteran diplomat Liu has been deputy head of the office since October last year.