The potential for China to use balloons for spying became a global issue in February when the United States shot down what it said was a Chinese surveillance balloon. China said the balloon was a civilian craft that accidentally drifted astray.

Taiwan is on high alert for Chinese military and political activity ahead of the Jan. 13 presidential and parliamentary elections. It says China is exerting military and economic pressure in an attempt to interfere in the elections.

China views the island as its own territory, a claim Taiwan's government rejects.

Since last month Taiwan's defence ministry has reported several instances of Chinese balloons flying over the sensitive Taiwan Strait. It said this week some balloons had flown over Taiwan island near major air bases.

In a statement on Saturday, the ministry said the balloons were a "serious threat" to international aviation safety given their flight paths.

"We also express our condemnation of the Chinese communists' disregard for aviation safety and its disregard for the safety of passengers on cross-Taiwan Strait and international flights," it said.

The ministry said its analysis was that the balloons were part of China's "grey zone" tactics against Taiwan "in an attempt to use cognitive warfare to affect the morale of our people".

Calls seeking comment to China's defence ministry, Taiwan Affairs Office and civil aviation administration were not answered on Saturday outside of work hours.

China's defence ministry last week declined to comment on the balloons at a monthly news conference.

STRONGER WORDING

In a separate statement on Saturday, Taiwan's defence ministry said that during the previous 24 hours it had detected two more Chinese balloons, one of which briefly flew over the far northern tip of the island.

The ministry had said it thought the balloons were mostly for weather monitoring, driven by prevailing winds at this time of year, and the comments on them being part of China's "grey zone" activities are stronger than previous statements.

Taiwan has complained over the past four years of stepped-up Chinese military action such as fighter jets regularly flying over the strait as part of a "grey zone" strategy attempting to wear down Taiwan with offensive actions that stop short of full-blown conflict.

The United States, Taiwan's most important international backer and arms supplier, has watched with alarm as tensions over the semiconductor powerhouse island have risen.

Asked about the balloons at a news conference in Washington on Thursday, White House national security spokesman John Kirby said he could not comment.

"We obviously support the democracy and the democratic institutions of Taiwan, and we want to see free, fair, open, transparent elections there." he said. "And we're certainly mindful that outside actors could try to interfere."

China says the Taiwan government's repeated accusations of election interference are "dirty tricks" aimed at boosting the chances of the ruling Democratic Progressive Party, which Beijing detests, calling them separatists.

A Western security source, speaking on condition of anonymity as they were not authorised to speak to the media, said China was sending a very simple pre-election message to Taiwan with the balloons.

"We are watching you closely and you can't hide," the source said.