Four Chinese and four Russian military planes entered South Korea's air defense identification zone(KADIZ) without notice on Tuesday.

Seoul's Joint Chiefs of Staff said the eight aircraft entered the KADIZ over southern and eastern waters in successive order between 11:52 a.m. and 1:49 p.m. before leaving the zone without violating South Korean airspace.

The JCS said it identified the planes even before they entered the KADIZ and sent Air Force fighter jets to the scene to prepare for a contingency.

Although the air defense zone is not territorial airspace, it is a buffer zone used to identify incoming aircraft for which it is standard practice to notify relevant countries in advance of flight plans that encroach upon the area.

The Chinese defense ministry said earlier that China and Russia held their sixth joint aerial strategic patrols over the East Sea and the East China Sea on Tuesday in line with their annual military cooperation plan.

The two countries have previously run joint drills or patrols that have crossed into the KADIZ, with the most recent instances occurring in May and November of last year, while China alone breached the zone with two planes in January.