South Korea scrambled fighter jets after two warplanes from China and four from Russia briefly entered an air identification zone maintained by Seoul, raising tensions on the highly militarized peninsula.

The Russian and Chinese aircraft were in South Korea’s Air Defense Identification Zone from between 11:53 a.m. and 12:10 p.m. on Thursday, Yonhap News agency reported, citing the Joint Chiefs of Staff. The six planes departed the area and did not enter into South Korean airspace, the JCS said, according to Yonhap.

The air zone isn’t South Korea’s territorial air space but is an area where aircraft are supposed to identify themselves as they draw near it. The planes flew above waters between South Korea and Japan, the JCS reportedly said, and were in the general vicinity of an island claimed by Seoul and Tokyo.

In June, four warplanes each from Russia and Chinese entered the area known as KADIZ, prompting South Korea to rush its jets into the air.

The flights come as the two Koreas have backed away from a 2018 deal to reduce tensions along their border. After North Korea fired a rocket into space last month that deployed a spy satellite, South Korea restarted surveillance flights it had halted and North Korea threatened action against the aircraft from its neighbor.

Russia has been looking to enlist North Korea in joint military exercises that include China. During a rare trip to Pyongyang in July, Russian Defense Minister Sergei Shoigu proposed to leader Kim Jong Un that they hold joint naval drills that included China, Yonhap reported, citing South Korea’s spy agency.

Read: Russia Proposed Joint Naval Drills With North Korea and China

There is no indication that North Korea accepted the offer. Kim’s state media on Thursday slammed the US, Japan and South Korea for their plan to launch a system this month to share real-time data on missile launches from North Korea, saying the move is pushing the region toward conflict.