TAIPEI, Taiwan (AP) — Taiwan’s Defense Ministry said that China sent four balloons over the island, three of which passed near to a key air force base.

The reported incursions on Tuesday come as China has been upping its threat to use force to annex the self-governing republic.

Taiwan is holding elections for its president and legislature on Jan. 13, and China has used its military, diplomatic and economic power to influence voters to back candidates favoring unification between the sides. Despite that, the pro-independence Democratic Progressive Party is leading in most polls, reaffirming the electorate’s backing for the status of de-facto independence from mainland China.

China regularly sends navy ships and warplanes to waters and airspace close to Taiwan, and its use of balloons to collect intelligence could be a new stage in its campaign of intimidation against the island.

The Defense Ministry said three of the balloons passed from east to west, close to the Ching-Chuan-Kang air base, home to much of the Taiwanese air wings dedicated to defending the island against China’s military threat. The fourth passed north of the port of Keelung, which services Taiwan’s crucial trade relations with Japan, a treaty partner with the U.S.

A Chinese balloon shot down by the U.S. after transferring North America last February was equipped to detect and collect intelligence signals as part of a huge, military-linked aerial surveillance program that targeted more than 40 countries, the Biden administration said, citing imagery from American U-2 spy planes.

A fleet of balloons operates under the direction of the People’s Liberation Army, the military wing of China’s ruling Communist Party, and is used specifically for spying, outfitted with high-tech equipment designed to gather sensitive information from targets across the globe, the U.S. said. Similar balloons have sailed over five continents, it said.