UN Secretary General Ban Ki-moon on Sunday urged North Korea on Sunday to stop launching missiles into the Sea of Japan, saying such actions were "provocative", dpa reported.
The South Korean news agency Yonhap on Sunday reported that the North had launched a fourth short-rage missile into the sea, citing the military. North Korea had fired three short-range guided missiles into the sea on Saturday.
"North Korea should stop its provocative actions including the launch of short-range guided missiles," the RIA-Novosty news agency quoted Ban as saying in Russia.
The reason for the launches remains unclear.
North Korea's missile and nuclear programmes are a source of international tension and sanctions, world powers have been closely watching whether Pyongyang has developed the know-now to mount a nuclear warhead onto a missile.
The United States warned North Korea last month that the world would not allow it to become a nuclear power.
They missile launches were carried out at a particularly tense time on the Korean Peninsula after North Korea in February conducted its third nuclear test, prompting new sanctions.
Pyongyang reacted with war threats.
In April, it also deployed two medium-range Musudan missiles on mobile launchers to its east coast, according to intelligence reports, but the United States said they were later withdrawn.