Chinese balloons became a politically fraught topic in February when the United States shot down what it called a spy balloon and Beijing said was a civilian airship blown off course.

Self-ruled Taiwan -- which China claims as its territory -- issued its first report of a balloon crossing the median line in the Taiwan Strait on December 8, then again this week, when it said two balloons were detected Sunday.

Another balloon was spotted "at 09:09 yesterday (0109 GMT Monday), around 67 nautical miles (124 kilometers) northwest of the city of Keelung", Taiwan's Ministry of National Defence reported Tuesday, referring to a northern coastal city.

It had an altitude of approximately 15,000 feet (4,752 metres) after crossing the strait.

"The balloon headed east and disappeared at 11:52," the statement added.

Defence Minister Chiu Kuo-cheng said after the first incident that it might be a weather balloon that had drifted towards Taiwan on monsoon winds and that it could be used for meteorological research and data.

Taiwan is on high alert ahead of presidential elections next month, with Taipei and Washington warning Beijing not to exert any influence over the vote.

Beijing, which has vowed to one day seize Taiwan, has ramped up political and military pressure since President Tsai Ing-wen took office in 2016.

This includes sending in warplanes and navy vessels around the island on a near-daily basis, which military experts say is a form of "intimidation".