The United Nations chief has voiced hope that North Korea will abandon its missile tests, after the Stalinist state fired three short-range missiles into the sea on Saturday.
Speaking in Russia Sunday, U.N. Secretary-General Ban Ki-moon called on Pyongyang to refrain from future launches and return to stalled nuclear talks with world powers.
Ban is in the Black Sea resort town of Sochi to meet with Russian President Vladimir Putin for talks on Syria.
Despite Ban's plea, Seoul said North Korea fired a fourth missile Sunday afternoon into the East Sea - also known as the Sea of Japan.
A South Korean defense ministry spokesman said on Saturday that the North's intent was not clear. He said South Korea's military was watching for any additional launches and "possible provocation."
Earlier this year, Pyongyang threatened nuclear strikes on South Korea and the U.S. in light of annual U.S.-South Korean military drills and U.N. sanctions imposed on the North after its third nuclear test in February.
About two months ago, North Korea fired two short-range missiles into the sea off its east coast. Pyongyang routinely conducts such exercises in an effort to improve its arsenal.
Some information for this report was provided by AP and AFP.