South Korea was scheduled to make an announcement on Sunday amid anticipation that it will expand its air defence zone south into a zone newly declared by China that has spurred regional tensions.
South Korea's defence ministry said the announcement at 0500 GMT/Midnight ET would be about its Air Defence Identification Zone (ADIZ), but declined to comment on the details.
South Korea has said China's move is unacceptable because its new zone includes the maritime rock named Ieodo which it controls, with a research station platform built atop it. China also claims the submerged rock as its own.
China's decision on Nov. 23 to declare an air defence zone in an area that includes islands at the center of a territorial dispute with Japan has triggered louder protests from Tokyo and Washington.
The decision was the subject of a tense disagreement as U.S. Vice-President Joe Biden visited China last week, stressing Washington's objections to the move that he said caused "significant apprehension" in the region.
Beijing said its zone was in accordance with international law and Washington and others should respect it.
Under the zone's rules, all aircraft have to report flight plans to Chinese authorities, maintain radio contact and reply promptly to identification inquiries.
U.S., Japanese and South Korean military aircraft have breached the zone without informing Beijing since it was announced. South Korean and Japanese commercial planes have also been advised by their governments not to follow the rules.
South Korea's reaction to the Chinese zone has been more measured than the protests from Tokyo and Washington. Officials have said they are reviewing a series of options that will ensure its national sovereignty is protected.