South Korea says it has extended its air defence zone to partially overlap with a similar zone newly declared by China that has spurred regional tensions.
South Korea's Defence Ministry said that the move would not infringe on neighbouring countries' sovereignty.
Earlier, three Chinese ships sailed through disputed waters off Tokyo-controlled islands in the East China Sea, according to the Japanese coastguard.
AAP reports it is the first such occurrence since Beijing announced an Air Defence Identification Zone over the sea.
Japan has vowed no compromise on sovereignty of the islands and stepped up defence spending, believing that China is trying to change the status quo through growing sea incursions.