Chinese balloons became a politically fraught topic in February when the United States shot down what it called a spy balloon over its territory, with Beijing saying the craft was a civilian airship blown off course.

Self-ruled Taiwan -- which China claims as its territory -- said two balloons crossed the median line in the Taiwan Strait on Sunday, after an earlier sighting on December 7.

Another balloon was spotted at 9:09 am local time (0109 GMT) on Monday, around 124 kilometres (77 miles) northwest of the coastal city of Keelung, Taiwan's defence ministry said Tuesday.

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

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

A ministry press officer said authorities had assessed that Monday's sighting was of a weather balloon, though did not elaborate on what the craft could be used for.

Defence Minister Chiu Kuo-cheng had said after the first incident that a weather balloon might have 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.