Taiwan's defense ministry has accused China of threatening aviation safety by flying balloons over its main island and surrounding waters.

The ministry said on Saturday that two balloons crossed the median line on the Taiwan Strait on Friday and one of them flew over the main island.

The ministry did not directly link the balloon incidents to Taiwan's presidential election to be held on January 13.

But it said they were part of China's "grey zone" tactics "in an attempt to use cognitive warfare to affect the morale of our people."

The ministry previously said it thinks that balloons are for weather monitoring, but indicated on Saturday that it does not rule out the possibility that Beijing is sending them for spying.

Chinese balloons have been detected for five straight days since the new year started.

The ministry says it has been aware that the number of balloons flying from China tends to increase in winter due to wind direction.

Last month it started releasing the number of balloons detected and their courses every 24 hours.