North Korea's attendance at China's biggest diplomatic event this year raised further eyebrows after Pyongyang fired a ballistic missile early on Sunday, in defiance of calls to rein in its weapons program.
Although the type of missile is not known, the USA had been expecting the North to fire a KN-17 medium-range ballistic missile, Martin reports.
Moon also ordered Seoul's military to maintain readiness, and to work closely with the U.S. to prepare for "any military provocation" by Pyongyang.
The launch of the as yet unidentified missile could signal advances over known North Korean capabilities, adding to tensions in the region as the Trump administration seeks to prevent Pyongyang from developing an intercontinental ballistic missile capable of reaching the US mainland.
At the same time, the US Pacific Command said that it had detected and tracked the North Korean missile launch, but there was no confirmation that it was an intercontinental ballistic missile.
"The North is apparently trying to test Moon and see how his North Korea policy as well as policy coordination between the South and the United States will take shape", said Yang Moo-Jin, professor at the University of North Korea Studies in Seoul.
"The flight altitudes of ICBMs are usually between 1,000 and 1,500 km, but this missile reached a maximum altitude of 2,000 km, meaning it could travel 6,000 to 7,000 km", the missile expert added, noting the North's missile could possibly target the Hawaii-based PACOM.
"North Korea's repeated firing of ballistic missile is a grave threat to the country (Japan), and a clear violation of the US Security Council's resolutions", Abe told the media ahead of Japan's national security meeting.
South Korea's Joint Chiefs of Staff said the missile was sacked early on Sunday morning from near Kusong, in North Phyongan province.
The U.S. believes the ballistic missile North Korea launched on Saturday was a KN-17 medium range missile and not a new intermediate range missile, as had been speculated.
North Korea's senior diplomat in charge of North America said Saturday the regime would talk to the Donald Trump administration "if the conditions were right", apparently responding to Trump's earlier comment he would be "honored" to talk to North Korean leader Kim Jong-un if the circumstances were right.
The North has conducted five nuclear tests despite United Nations sanctions and is also developing long-range missiles.
In an interview with CBS News' "Face the Nation" in April, President Trump said that pursuing China's help with North Korea is more important than securing better trade agreements. The USS Carl Vinson, an aircraft supercarrier, is also engaging with South Korean navy ships in waters off the Korean Peninsula, according to Seoul's Defense Ministry.
Last week South Koreans elected Moon to replace conservative Park Geun-hye, who is in jail awaiting a corruption trial.
South Koreans watch a television displaying news broadcasts reporting on North Korea's recent ballistic missile launch, at a station in Seoul, South Korea.
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