South Korea has expressed its concern to relevant international organizations over North Korea's recent ballistic missile launches, putting further pressure on the communist regime, a government source said Sunday.

In a letter to the International Civil Aviation Organization and the International Maritime Organization, South Korea said the North's launchings of short- and medium-range ballistic missiles earlier this month were "dangerous" as they came without prior warning.

Under international conventions, North Korea is required to inform relevant organizations before launching missiles to ensure the safety of civilian aircraft and vessels passing through the area.

ICAO raised the issue with North Korea following the short-range missile launches on March 10. Both ICAO and IMO are expected to lodge another protest with the North after reviewing South Korea's concern and separately looking into the medium-range ballistic missile launches that took place Friday.

Later that day, the United Nations Security Council "strongly condemned" the North over the missile tests, saying they constituted a "clear violation" of UNSC resolutions that ban the country from using ballistic missile technology.

In early March, the UNSC slapped tougher sanctions on North Korea to punish the regime for its fourth nuclear test on Jan. 6 and long-range rocket launch on Feb. 7. (Yonhap)