North Korea fired 10 short-range missiles into the sea off the east of the Korean peninsula on Sunday, South Korea's Yonhap news agency reported, citing unidentified government officials in South Korea.
Yonhap said the missiles flew for 70 km (45 miles) before splashing into the sea.
North Korea is not banned from short-range missile launches under U.N. sanctions and frequently tests its arsenal.
The U.S. State Department said it was closely monitoring the situation after the reports of the missile firing.
"We once again call on North Korea to refrain from provocative actions that aggravate tensions," State Department spokeswoman Jen Psaki said in a brief statement.
The North has toned down its bellicose rhetoric towards the South, although it has denounced joint U.S.-South Korean military exercises that are currently taking place.
(Read more: China draws 'redline' on North Korea)
It undertook a series of short-range missile launches earlier this month as the joint exercises, which North Korea views as a threat to its security, started.
In early 2013, North Korea conducted its third nuclear weapons test, having successfully launched a long-range rocket in 2012 that critics say was aimed at proving technology for an intercontinental ballistic missile.
Both of those are banned under U.N. sanctions.