Yonhap said the missiles were surface-to-surface KN-02 rockets with a range of up to 120km.
The report said North Korea had also issued a no-sail zone in an area off the east coast until October 20.
South Korea's defence ministry said it could not confirm the report.
International talks
The reported missile tests are the first since North Korea launched a series of missiles in July.
They come despite signs that North Korea is willing to return to six-party talks with South Korea, China, Japan, Russia and the US on its nuclear weapons programme after months of tensions.
Earlier this month, North Korea's official KCNA news agency quoted Kim Jong-il, the country's leader, as saying the North was ready to return to multilateral talks, but wanted direct negotiations with the US first.
The North had walked away from the talks in April and declared its agreements invalid after the UN condemned Pyongyang's long-range rocket launch.
Reacting to reports of the missile launches on Monday, Hillary Clinton, the US secretary of state, said the US would continue to work towards a nuclear-free Korean peninsula.
"Our goals remain the same. We intend to work toward a nuclear free Korean peninsula," Clinton told a news conference.
"Our consultations with our partners and our allies continues unabated. It is unaffected by the behaviour of North Korea.
"We have made a lot of progress with the other members of the six-party talks who joined us in the very strong sanctions against North Korea and who have been working with us to restart a process there."