Eleven Chinese aircraft crossed the median line of the Taiwan Strait on Friday morning, an unofficial barrier between the two sides, Taiwan's defence ministry said, as China ramps up military pressure on the democratic island.

Taiwan, which China claims as its own territory despite the island's objections, has complained in recent years of almost daily missions by the Chinese air force, often in the southwestern part of its air defence identification zone.

A total of 24 Chinese war planes including fighter jets and bombers were spotted near Taiwan on Friday morning from around 8 am (00:00 GMT), the defence ministry said, adding five Chinese battleships also joined a "joint war readiness patrol."

Taiwan sent aircraft and ships to warn away the Chinese activities, while missile systems monitored them, the ministry said, using standard wording for its response.

A senior Taiwan security official told Reuters the Chinese activities on Friday were likely a reaction to the visit to Taipei made by a US congressional delegation led by the chair of House Armed Services Committee Mike Rogers.

China, which has never renounced the use of force to bring Taiwan under its control, in April staged drills around the island in anger at President Tsai Ing-wen's meeting with the speaker of the US House of Representatives.

Taiwan's government strongly objects to China's sovereignty claims and vows to defend itself if China attacks the island.

The US State Department on Thursday approved the potential sale of ammunition and logistics supply support to Taiwan in two separate deals valued at up to $440 million.