North Korea needs tests to ideal its missile program, but it also is thought to time its launches to come after the elections of new US and South Korean presidents in what analysts say are efforts meant to gauge a new administration's reaction.
One recent missile launch staged in March saw three North Korean missiles falling provocatively close to Japan, sparking alarm in Tokyo.
The South Korean president has repeatedly pledged to seek dialogue with North Korea, having served as chief of staff when the last inter-Korean summit took place in 2007.
The launch, if it is confirmed to be test-firing of a ballistic missile, would be the first in two weeks since the last attempt to fire a missile ended in a failure just minutes into flight.
Mr Moon called the launch a "grave challenge" to the peace and security of the Korean Peninsula and the worldwide community.
"China opposes relevant launch activities by North Korea that are contrary to Security Council resolutions", China's foreign ministry said in a statement sent to Reuters.
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It impacted "so close to Russian soil ... the president can not imagine that Russia is pleased", the White House said. Pyongyang defends its missile and nuclear programs as means of protecting the country from USA hostility.
"At this point, we see nothing consistent with an (intercontinental ballistic missile) launch", a second defense official told CNN.
The White House said: "With the missile impacting so close to Russian soil - in fact, closer to Russia than to Japan - the President can not imagine that Russia is pleased".
"And, in the short term, North Korea's missile capability is still some distance from Washington's red line - where it presents a real threat to USA territory".
Moon condemned the North's latest missile launch as a "reckless provocation", staged days after his inauguration in an apparent test of the new administration.
The forum also raises the possibility of officials from North Korea, China, United States, Russia, South Korea and Japan holding informal discussions on the sidelines of the event.
North Korea launched the Pukguksong-2 missile, an upgraded, extended-range version of its submarine-launched ballistic missile (SLBM), from the same Kusong site on 12 February. While Pyongyang regularly tests shorter-range missiles, it is also working to master the technology needed to field nuclear-tipped missiles that can reach the USA mainland.
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In Japan, Prime Minister Shinzo Abe said North Korea's missile launches were a "grave threat to our country and a clear violation of United Nations resolutions".
But Jeffrey Lewis, director of the East Asia nonproliferation program at the Middlebury Institute of International Studies at Monterey in California, said at the time that there was an "even chance" that they were intercontinental ballistic missiles (ICBMs).
Chief Cabinet Secretary Yoshihide Suga said North Korea's firing of a ballistic missile was a violation of United Nations resolutions and that Japan strongly protested the action.
Unlike his conservative predecessors, Mr Moon advocates reconciliation with Pyongyang but warned on Sunday that dialogue would be possible "only if the North changes its attitude".
Pyongyang's test launch of a long-range missile tests appears to have deeply upset almost everyone with a stake in the current nuclear stand-off, including ally Beijing, arch-rival Seoul, an increasingly belligerent Washington, long-time adversary Tokyo and former friend Moscow.
It represented a more significant threat because of the difficulty of tracking a mobile launcher and because of the ability to keep the missile fueled, unlike liquid fuel rockets.
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