China said Friday it was conducting military exercises near the Taiwan Strait, as a top US 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.

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 mounted a decades-long policy of marginalising the democratic island.

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

At a press conference on Friday morning a Chinese defence ministry spokesman said Beijing was "holding actual combat exercises near the Taiwan Strait" when asked how Beijing would respond to Krach's 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 army 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.

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".

In recent weeks, Taiwan has reported a sharp rise in incursions by Chinese jets into its air defence identification zone.

Washington's increased outreach to Taiwan under US President Donald Trump has become yet another flashpoint between China and America, with US cabinet member Alex Azar visiting last month.

Washington said Krach was visiting Taiwan to attend a memorial service for late president Lee Teng-hui. He will also meet with foreign minister Joseph Wu, and is scheduled to join President Tsai Ing-wen for dinner at her official residence Friday.