"We first believed they were an improved version of the KN-02 ground-to-ship missile, but we now believe they are Scud series missiles as they have a range of more than 200 kilometers [125 miles]," the source told Yonhap.
The South Korean Defense Ministry said the missiles were fired Thursday by the Communist country from its southeast coast as the annual U.S.-South Korean military drills were in progress. The drills will continue until April.
The North has opposed the drills, saying they are a rehearsal for an attack against it, although the South has assured they are routine and defensive in nature.
Yonhap said if the Scud report is confirmed it would be the North's first firing of a Scud missile since July 2009.
South Korean officials said they have increased their vigilance and readiness against additional missile launches and other possible provocations.
In Washington, State Department spokeswoman Jen Psaki, commenting on the missile launches, said the United States is "of course closely monitoring the situation on the Korean Peninsula" and that it continues to urge North Korea to exercise restraint and take steps to improve its relations with its neighbors.
"As you know, the United States works very closely with the international community and our partners in the region to address global security and proliferation threat -- the proliferation threat posed by North Korea," she said.
Officials told Yonhap the North has three types of Scud missiles -- the Scud B with a range of 300 kilometers (186 miles), the Scud C with a range of 500 kilometers (310 miles) and Scud D with a range of 700 kilometers (434 miles).
Yonhap quoted a military official as saying more analysis is needed to determine the exact type of the projectiles, saying they could also be improved versions of the KN-02 or new rockets of at least 300mm in caliber fired from a multiple rocket launcher.
The North's missile and nuclear programs have aggravated tensions on the Korean Peninsula. The North conducted its third nuclear test a year ago.