China’s foreign ministry said it was «strongly» opposed to the planned sale, adding that the United States should stop arms sales and military contacts with Taiwan.

The United States is required by law to provide Taiwan with defensive weapons, but has a policy of «strategic ambiguity» when it comes to exactly how it would respond to Chinese aggression toward the self-governing island. Since establishing diplomatic relations with China in 1979, the US has recognized Beijing as China’s sole legitimate government while maintaining unofficial relations with Taiwan.

Chinese President Xi Jinping has not ruled out the use of force to achieve his goal of «reunification» with Taiwan. Taiwan has complained for the past three years about increased Chinese military activities near the island as Beijing seeks to assert its sovereignty claims.

Taiwan on Thursday reported a second day of large-scale Chinese air force raids on its air defense identification zone, with its defense ministry saying it had seen 21 aircraft over the past 24 hours.

China has said its activities in the area are justified as it seeks to defend its territorial integrity and warn the United States against «collusion» with Taiwan, despite the anger this causes in Taipei.

Taiwan’s Defense Ministry said the aircraft, 17 J-10 fighters and four J-16 fighters, had flown into the southwest corner of Taiwan’s air defense identification zone, according to a map released by the ministry.

Taiwanese forces monitored the situation, including dispatching their own planes, the ministry added, using the normal wording for its response to such Chinese incursions.

The ministry on Wednesday reported 19 Chinese planes flying in Taiwan’s air defense zone.

Neither plane crossed the delicate median line of the Taiwan Strait, which has served as an unofficial barrier between the two sides but which China’s air force has been flying over almost daily since it held war games near Taiwan in last august.

Taiwan last reported a large median line crossing by Chinese aircraft on Friday, when 10 aircraft were involved.

China has not commented on recent activities near Taiwan. In January, China said it held combat exercises on the island to «resolutely counter the provocative actions of external forces and Taiwanese separatist forces.»

No shots have been fired and the Chinese planes have been flying Taiwan’s ADIZ, not its territorial airspace.

ADIZ is a larger area that Taiwan monitors and patrols, giving it more time to respond to any threats.

Taiwan’s government has repeatedly offered talks with China, but says the island will defend itself if attacked and only the Taiwanese people can decide their own future.