Taiwan's defense ministry condemned Beijing on Saturday for sending balloons across the median line that separates the self-ruled island and China, saying they pose a "serious threat" to aviation routes and are a form of harassment. 

China claims Taiwan as part of its territory and has in recent years stepped up military and political pressure — sending in an unprecedented number of fighter jets and naval vessels around the island. 

In December, Taipei's defense ministry began reporting the appearance of Chinese balloons around Taiwan, noting at least four incidents in which they flew directly above the island. 

"Based on their recent drift path, (the ballons) have posed a serious threat to the safety of many international aviation routes," the Ministry of National Defence said in a statement condemning China's "disregard for aviation safety".

The ministry also released an illustration Saturday showing two Chinese balloons it said had crossed the median line a day before, with one directly above the island.

"(The) main purpose of the recent detection of balloons is 'grey zone' harassment in an attempt to use cognitive warfare to affect the morale of our people," it added. 

"Grey zone" tactics refer to aggressive actions deployed by a state that stop short of open warfare — which many experts say is what China is doing to Taiwan with its near-daily show of military force around the island. 

"We call for an immediate end to this practice to ensure flight safety in the region," the ministry said

The statement comes a week before Taiwan's presidential election, which will determine the future of the island's relations with Beijing depending on who voters pick. 

Since the election of Taiwanese President Tsai Ing-wen in 2016, China has severed all high-level communications with her government as she does not recognize Beijing's sovereignty over the island.