As an election in democratic Taiwan looms, Chinese balloons have appeared around the island at various times of day in what its military says is a new form of "grey zone" harassment.

China already flexes its military prowess against self-ruled Taiwan daily, sending fighter jets, reconnaissance drones, and naval ships around the island.

But since December, Taiwan's defense ministry has said that mysterious balloons have crossed the narrow Taiwan Strait separating the island from China in a kind of "grey zone" harassment.

"Grey zone" tactics are aggressive actions deployed by a state that stops short of open warfare. Experts say that is what China has been doing with its show of military force around the island. 

Days before a presidential election, some balloons have flown directly above Taiwan, with at least one detected around a military airbase.

"Beijing is signaling that it can violate Taiwanese airspace at will and challenge its sovereignty," said Raymond Kuo, Taiwan Policy Initiative director at RAND Corporation. 

Self-ruled Taiwan is claimed by China as part of its territory, with Beijing's leadership vowing to bring the island under its control. 

The 13 January election pits frontrunner Lai Ching-te — whose ruling Democratic Progressive Party says Taiwan is a sovereign nation — against two others who have pledged warmer relationships with Beijing. 

The timing of the balloons' appearance is "political", said Kuo. 

"The CCP (Chinese Communist Party) is attempting to demoralize the Taiwanese public in the run-up to next week's presidential and legislative elections," he told AFP. 

But its military purpose is "in line with broader China's grey zone coercion strategy towards Taiwan". 

"Beijing is adding yet another operational challenge to Taiwan's defense forces, which could erode readiness and resilience," he said.