Taiwan's Defence Ministry says a Chinese military surveillance balloon was spotted in the Taiwan Strait, as well as a large-scale movement of military aircraft and ships.

The ministry said the balloon passed southwest of the northern port city of Keelung on Thursday night, then continued east before disappearing, possibly into the Pacific Ocean.

However, there seemed to some uncertainty about whether the balloon was operated directly by the People's Liberation Army, the military branch of China's ruling Communist Party. The ministry referred to it both as a "PLA surveillance balloon" and as "1 PRC's balloon", using the acronym for the People's Republic of China, China's official name.

A Defence Ministry spokesperson said they had no additional information.

China has long blurred the lines between military and civilian functions, including in the South China Sea, where it operates a huge maritime militia — ostensibly civilian fishing boats that act under government orders to assert Beijing's territorial claims.

Taiwan has threatened to shoot down such balloons, but the ministry did not say what, if any, action was taken. It said the balloon was monitored flying at an altitude of approximately 6,400 metres (21,000 feet).

It also said 26 Chinese military aircraft were detected, along with 10 Chinese navy ships, in the 24 hours before 6 am on Friday.

Of the aircraft, 15 had crossed the median line that is an unofficial divider between the sides, but which Beijing refuses to recognise. Some also entered Taiwan's self-declared air defense identification zone outside the island's airspace, which encompasses the 160 kilometer (100 mile) - wide Taiwan Strait.

Taiwan's military monitored the situation with combat aircraft, navy vessels and land-based missile systems, the ministry said. The incident came just over month before Taiwan is to hold elections for president and the legislature and raises questions about possible Chinese efforts to influence the vote.

Such incursions occur regularly as a means of advertising China's threat to use force to annex the self-governing island republic it considers its own territory, wear down Taiwan's military capabilities, and impact morale among the armed forces and the public, who remain largely ambivalent to China's actions.

The Chinese missions have also prompted Taiwan to up its purchases of aircraft from the United States, its chief ally, and revitalize its own defence industry, including producing submarines.

Beijing strongly protests all contacts between the island and the US, but its aggressive diplomacy has helped build strong bipartisan support for Taipei on Capitol Hill.

US President Joe Biden vowed sharper rules to track, monitor and potentially shoot down unknown aerial objects after three weeks of high-stakes drama sparked by the discovery of a suspected Chinese spy balloon transiting much of the country early in the year.

The US labelled the balloon a military aircraft and shot it down with a missile and recovered what it said was sophisticated surveillance equipment it was transporting. China responded angrily, saying it was merely a weather balloon that had blown off course and called the shoot-down a major overreaction.