"We continue to face a grave situation due to a series of provocations and nuclear threats by North Korea," said South Korean Foreign Minister Yun Byung-se at an annual conference of chiefs of overseas diplomatic missions in Seoul.
Analysts suggested that the latest missile launches were part of military drills.
Pyongyang routinely test-launches such missiles, but the latest tests followed several weeks of relative calm on the Korean Peninsula after tensions escalated earlier this year, following North Korea's test of a long-range Taepodong 2 missile in December and its third nuclear test in February, to which the UN responded with sanctions.
UN Secretary General Ban Ki-moon said Sunday he was concerned over North Korea's latest short-range missile launches and called on Pyongyang to return to six-party talks on its controversial nuclear programme.
North Korea has been subjected to several rounds of UN Security Council (UNSC) sanctions since declaring itself a nuclear power in 2005.
The reclusive communist regime broke off talks with South Korea, China, the US, Japan and Russia on its nuclear programme in 2009, after the UNSC passed a resolution condemning its missile tests.