Four Chinese balloons were detected moving across the median line separating Taiwan from China, with three flying directly above the island, Taiwan's defense ministry said Wednesday.

A graphic released by the ministry showed three balloons headed northeast directly above the island on Tuesday after appearing "southwest of Ching-Chuan-Kang", home to a military airbase in Taiwan's western Taichung city. 

The lowest altitude measured for one of them was 12,000 feet (3,658 meters). 

Tuesday's balloon incidents came less than two weeks before the self-ruled island will hold a pivotal election, the second consecutive day such objects were detected above Taiwan. 

The ministry began reporting them last month for the first time, with six incidents in December. 

Conflict expert Ou Sifu of Taiwan's Institute of National Defense and Security Research said the balloons are "for military coercion and psychological warfare".  

"The presidential election is coming and balloons are a kind of military intimidating tool," he told AFP, adding that China wished to influence more "pro-Beijing votes". 

Beijing claims Taiwan as part of its territory and has in recent years ramped up military and political pressures against the government of President Tsai Ing-wen. 

It has sent in unprecedented numbers of warplanes and naval vessels around the island, dubbed by conflict experts as "grey zone" tactics that serve to intimidate Taiwan's military. 

Beijing has also refused to engage with Tsai during her tenure, as she does not acknowledge China's claim on Taiwan. 

Her deputy Lai Ching-te — a self-proclaimed "pragmatic worker for Taiwan's independence" — is the current frontrunner for the 13 January election. 

Beijing has dubbed him a "troublemaker" and a "separatist".