Taiwan’s air force was scrambled again on Monday to warn away 13 Chinese aircraft that had entered its air defense zone, according to Taiwan’s defense ministry, in the latest flare-up in tensions across the Taiwan Strait.

Taiwan, which China claims as its own territory, has complained about the Chinese air force conducting such missions on a regular basis over the last two years, despite the fact that the aircraft do not fly close to Taiwan.

Taiwan is currently on high alert due to fears that China will use Russia’s invasion of Ukraine to launch a similar military attack on the island, despite the fact that Taipei’s government has not reported any unusual Chinese movements.

The number of aircraft involved was well off the last large-scale incursion, 39 Chinese aircraft on Jan. 23, and since then, such fly-bys have been sporadic with far fewer aircraft.

The ministry said the latest mission involved seven Chinese J-10 and five J-16 fighters as well as one Y-8 electronic warfare aircraft, which flew over an area to the northeast of the Taiwan-controlled Pratas Islands at the top end of the South China Sea.

Taiwanese fighters were sent up to warn the Chinese aircraft and air defense missiles were deployed to “monitor the activities”, the ministry said, using standard wording for how Taiwan describes its response.

The incident happened on the same day Taiwan grounded its fleet of Mirage 2000 fighters after one crashed on a training mission, though its other jets including F-16s remain active.

China has never renounced the use of force to bring Taiwan under its control, and the Taiwan Strait remains a potentially dangerous military flashpoint.