The island’s defense ministry said on Monday that two suspected Chinese weather balloons crossed the delicate Taiwan Strait on Sunday but stayed far to the north of Taiwan. Taipei has reported seeing them close to the weather balloons twice this month.

China claims that the balloon it shot down in February was a civilian craft that had accidently drifted astray, while the United States claimed it was a Chinese surveillance balloon, raising concerns about China’s propensity to employ balloons for espionage around the world.

Taiwan is on high alert for Chinese activities, both military and political, ahead of the Jan. 13 presidential and parliamentary election. Taipei has warned of Beijing’s efforts to interfere in the ballot to get voters to pick candidates China may prefer.

Taiwan’s defence ministry said the two balloons were detected at 9:03 a.m. (0103GMT) and 2:43 p.m. (0643) after crossing the strait’s median 110 nautical miles (204 km)northwest of the northern Taiwanese port city of Keelung.

The balloons flew at an altitude of about 27,000 feet (3,230 metres), headed east and disappeared at 9:36 a.m. and 4:35 p.m., respectively, the ministry added.

The ministry’s initial judgement is that they were weather balloons, spokesperson Sun Li-fang said.

China’s defence ministry did not immediately respond to a request for comment.

The earlier balloon, which Taiwan reported crossing the Taiwan Strait on Dec. 7, was most likely also a weather platform, Taiwan’s defence ministry said at the time, adding that officials had announced its detection in the interests of transparency.