Taiwan’s air force was rushed again on Thursday to scare away nine Chinese aircraft that had breached its air defense zone, according to Taiwan’s defense ministry, on the same day Russia attacked Ukraine, a crisis that Taipei is keenly monitoring.

Taiwan, which China claims as its own territory, has protested about the Chinese air force conducting such flights on a regular basis during the previous two years, despite the fact that the aircraft do not fly near to Taiwan.

The number of aircraft participating was significantly less than the previous large-scale intrusion, which comprised 39 Chinese aircraft on Jan. 23, and similar fly-bys have been intermittent with far fewer aircraft since then.

The ministry said the latest mission involved eight Chinese J-16 fighters and one Y-8 reconnaissance 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.

Taiwan has been warily watching the Ukraine crisis, nervous that China may try to take advantage to move on the island.

While Taipei has not reported any unusual movements by Chinese forces, the government has increased its alert level.

China has never renounced the use of force to bring Taiwan under its control, and routinely condemns U.S. arms sales or other shows of support from Washington.

Speaking in Beijing earlier on Thursday when asked about the new U.S. Indo-Pacific strategy, Chinese Defense Ministry spokesman Tan Kefei reiterated that Taiwan was a “core issue” of China’s and it would tolerate no foreign interference.

“We urge the U.S. side to recognize the high sensitivity of the Taiwan issue, stop interfering in China’s internal affairs and stop playing with fire on the Taiwan issue,” Tan said.

In the 12-page Indo-Pacific strategy overview issued earlier this month, the Biden administration vowed to commit more diplomatic and security resources to the region.

On Taiwan, Washington would work with partners inside and outside the region to maintain peace and stability in the strait dividing the island from China, it said.