South Korea says it has extended its air defence zone to overlap in part with a similar zone declared recently by China.
Beijing’s new zone including islands claimed by Japan has brought international protests.
South Korea says its move does not infringe on neighbouring countries’ sovereignty.
“This modified Korean Air Defence Identification Zone is in line with the international aviation order and international regulations,” said South Korean Defence Ministry spokesman Kim Min-seok.
US B-52 bombers have already tested the Chinese zone.
South Korea’s reaction has been more measured; Beijing is Seoul’s largest trading partner.
The new extended zone will come into effect next weekend and the defence ministry says it will not affect commercial flights.
Chinese television has shown pictures of its country’s own military drills in the East China Sea, where rival territorial claims have strained relations with its neighbours.
Last week Beijing warned South Korea to conform to international law, insisting its own defence zone was legal.
All foreign aircraft are expected to submit to demands by Chinese authorities, rules that have been ignored by the US, Japan and South Korea.