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.