The launch, if it is confirmed to be test-firing of a ballistic missile, is the first in two weeks since the last attempt to fire a missile ended in a failure just minutes into flight.
The South Korean military said an "unidentified projectile" had been fired from Kusong, but did not give details.
That missile, launched April 29, blew up over land in North Korean territory, according to a spokesman for the U.S. Pacific Command.
Choi did not elaborate on what the North's conditions are, but her comments raise the possibility of North Korea and the USA returning to negotiations for the first time since 2008, when six-nation talks over Pyongyang's nuclear weapons program fell apart. "North Korea's missile launch is a serious threat to Japan and clearly violate against the United Nations resolution".
The launch is the first since a new liberal president took office in South Korea on Wednesday saying dialogue as well as pressure must be used to ease tensions on the Korean peninsula and stop the North's weapons pursuit.
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Moon called an emergency national security meeting Sunday, but he didn't immediately make any statement on the launch.
The weapon was sacked from Kusong, located To the northwest of the North Korean capital of Pyongyang.
Earlier this month Trump said he would be willing to meet North Korean leader Kim Jong Un "under the right circumstances".
Moon, the first liberal leader in Seoul in almost a decade, said as he took his oath of office that he'd be willing to visit the North if the circumstances were right. But analysts say that North Korea's weapons program is likely too far advanced for him to be able to return to the type of sunshine policies his liberal predecessors favored.