North Korea has test launched an additional two short-range projectiles into the sea from a site near the eastern city of Wonsan, Kangwon Province.
A South Korean Joint Chiefs of Staff (JCS) statement was quoted by Yonhap News Agency as saying that the projectiles were believed to be the new 300mm multiple-rocket launcher KN-09 shells, and were fired in a north-eastern direction.
"Our military has maintained vigilance against additional launches and is taking necessary steps," the statement said.
An unnamed JCS official said the rockets flew 180km, and are believed to have landed in North Korea's territorial waters, not international waters.
"It is yet to be confirmed if the North made a pre-announcement for civilian flights or vessels. We've just detected them with our military surveillance equipment," the official said.
The 300mm rocket poses a threat to South Korea, as it is able to strike major military facilities, including the headquarters of three military branches that are located 130km south of Seoul, according to the Yonhap News Agency.
The launches were undertaken a day before Chinese President Xi Jinping's visit to South Korea.
North Korea successfully tested its 'newly developed tactical guided missile' on 26 June in the presence of its leader Kim Jong Un, and followed it up with two short-range ballistic missiles, presumed to be Scuds with a range of approximately 500km, on 29 June.