TAIPEI (Reuters) – Taiwan’s air force warned off several Chinese fighter jets that briefly entered Taiwan’s air defence identification zone to its southwest on Tuesday, the defence ministry said.

The Su-30 fighters, some of China’s most advanced jets, were given verbal warnings to leave and Taiwanese air force jets “drove away” the intruders, ministry added.

Taiwan has complained that China, which claims the democratic island as its own, has stepped up military activities in recent months, menacing Taiwan even as the world deals with the coronavirus pandemic.

China says such exercises are nothing unusual.

China has never renounced the use of force to bring Taiwan under its control. One of China’s most senior generals last month said China would attack if there was no other way of stopping Taiwan becoming independent.

China is deeply suspicious of Taiwanese President Tsai Ing-wen, whom it accuses of being a separatist bent on declaring formal independence. Tsai says Taiwan is already an independent country called the Republic of China, its official name.

The United States has stepped up its military activities near the island too, with semi-regular navy voyages through the narrow Taiwan Strait.

A U.S. C-40A, a military version of the Boeing 737, had entered Taiwanese air space with permission, though it did not land at any Taiwanese airports, Taiwan’s Defence Ministry said in a separate statement on Tuesday.

The U.S. aircraft took off from Japan’s Okinawa island, where there is a major U.S. air base, and flew over northern and western Taiwan on its way to Southeast Asia, Taiwanese media reported.

While Washington and Taipei have no formal diplomatic ties, the United States is Taiwan’s strongest international supporter and main arms supplier, becoming another source of U.S.-China tension.