China said Friday it was conducting military exercises near the Taiwan Strait, as a top U.S. diplomat visits the self-ruled island in a move that has angered Beijing.

Relations between the United States and China are at their lowest point in decades, with the two sides clashing over a range of trade, military and security issues, as well as the coronavirus pandemic.

Keith Krach, U.S. undersecretary of state for economic growth, energy and the environment, landed in Taipei Thursday for a three-day visit, the highest-ranking State Department official to visit in 40 years.

At a press conference on Friday, a Chinese defense ministry spokesman said Beijing was "holding actual combat exercises near the Taiwan Strait" when asked how it would respond to the visit.

"This is a legitimate and necessary action taken to safeguard China's sovereignty and territorial integrity in response to the current situation in the Taiwan Strait," Ren Guoqiang told reporters.

Ren also warned that the Chinese military had "sufficient ability" to counter any external threat or challenge from Taiwan separatists.

Beijing considers Taiwan part of its territory, to be absorbed into the Chinese mainland -- by force if necessary.

China's Communist leadership baulks at any recognition of Taiwan -- which has been ruled separately from China since the end of a civil war in 1949 -- and has pursued a decades-long policy of marginalizing the democratic island.

Ren accused the United States of "frequently causing trouble" over Taiwan, which he said "is purely China's internal affairs, and we won't tolerate any external interference".

According to Taipei's defense ministry, 18 Chinese aircraft -- including bombers and fighters -- entered Taiwan's southwest air defense identification zone (ADIZ) on Friday and also crossed the so-called median line that divides the Taiwan Strait.

The ministry said Taiwan's military "scrambled fighters, and deployed air defense missile system to monitor the activities".

"We hope the other side can exercise restraint and not... heighten conflicts between the two sides. These military intimidations have caused resentment among the Taiwanese people," it said in a statement.

In recent weeks, Taiwan has reported a sharp rise in incursions by Chinese warplanes into its ADIZ.

Chinese jets also made a brief incursion across the midline of the strait separating the two sides in August, as US health chief Alex Azar made his country's highest-level visit to Taiwan since 1979 -- the year Washington switched diplomatic recognition to Beijing.

Washington's increased outreach to Taiwan under President Donald Trump has become yet another U.S.-China flashpoint.

The U.S. said Krach was visiting Taiwan to attend Saturday's memorial service for late president Lee Teng-hui, who died in July aged 97.

On Friday, Krach met with foreign minister Joseph Wu to discuss various bilateral issues and exchange view on future collaborations, according to Taipei authorities. He is also scheduled to join President Tsai Ing-wen for dinner at her official residence.

China has ramped up pressure on Taiwan since Tsai came to power in 2016, as she refuses to acknowledge its idea that the democratic island is part of "one China".