South Korea is to announce a new air defence zone in the growing row with China over contested airspace above the East China Sea.
South Korean security agencies and ministries were working on declaring the fresh zone to expand the country's southern territories.
A key ocean research station on a submerged reef was expected to be included in the zone, according to local reports.
"Once the plan is finalised, we will explain the meaning of the new air defence zone to the US, China and Japan. As a responsible member of the international community, the South Korean government will try to give enough explanation to neighbouring nations so as not to undermine trust with them," a senior South Korean government official told the state-run Yonhap news agency.
The announcement was made on the eve of a visit by US vice-president Joe Biden. He kicked off his Asia tour in Japan en route to South Korea. After meeting Japanese Prime Minister Shinzo Abe, Biden urged both Tokyo and Beijing to lower tensions in the region over a disputed group of islands known as Diaoyu in China and Senkaku in Japan.
The long-simmering dispute bubbled up again recently with China's unilateral decalration of the Air Defence Identification Zone over the archipelago.
"We remain deeply concerned by the announcement of the new Air Defence Identification Zone," said Biden. "This latest incident underscores the need for agreement between China and Japan to establish crisis management and confidence building measures to lower tensions."