The launches come three days after South Korea said the north fired a medium-range missile into the sea for the first time since 2014.
Washington and Seoul are conducting joint military drills, which North Korea sees as a rehearsal for invasion.
The US last week imposed new sanctions on the north.
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The new sanctions were in response to North Korea's nuclear test on 6 January and its satellite launch on 7 February, which violated existing UN sanctions.
South Korea's military was trying to find out what types of missile had been involved in the latest launches, Yonhap reported.
The missiles were said to have been fired from the northeastern city of Hamhung.
On Friday North Korea test-fired two ballistic missiles.
US officials said the medium-range missiles, launched off the east coast, flew about 800km (500 miles) before falling into the water.
Afterwards, the US called for Pyongyang to refrain from raising tensions.
North Korea has fired several short-range missiles into the waters off its east coast, South Korea's Yonhap news agency says.