The US has urged North Korea to exercise restraint after reports it fired two short-range missiles into the sea off its east coast.
US state department spokeswoman Jen Psaki said North Korea should refrain from "provocative actions that aggravate tensions".
South Korean defence officials reported the tests, which came amid joint South Korean-US military exercises.
Pyongyang also test-fired four similar missiles late last week.
South Korean officials said that the missiles launched early on Monday morning were Scud-C models, which have the range to hit any target in South Korea.
Officials in Seoul said the missiles flew more than 500km (300 miles).
North Korea often carries out short-range missile tests to express its anger at its southern neighbour's participation in military exercises.
South Korea and the US began the annual joint exercises on 24 February. The drills, which involve thousands of troops, continue until the middle of April.
Pyongyang is opposed to the drills and has previously called them "exercises of war".
Last year North Korea reacted furiously to the annual military exercises, which came shortly after Pyongyang conducted its third nuclear test, prompting the expansion of existing UN sanctions.
This year, however, ties between the two Koreas have warmed somewhat. Earlier this month, they held reunions for families separated by the division of the Korean peninsula for the first time since 2010.
"The North is taking a double-faced stance by making conciliatory gestures on one hand and pushing ahead with reckless provocation on the other," said South Korea's Defence Ministry spokesman Kim Min-seok.