US President Donald Trump has cancelled a summit with North Korean leader Kim Jong-un, saying the world had "lost a great opportunity for lasting peace".
He said his decision was because of "tremendous anger and open hostility" in a recent North Korean statement.
The summit aimed to rid the Korean peninsula of nuclear weapons and would have been the first time a sitting US president met a North Korean leader.
But both sides recently cast doubt on whether the talks would happen.
Mr Trump's announcement came just hours after North Korea said it had dismantled tunnels at its only nuclear test site in a move witnessed by foreign reporters.
South Korean President Moon Jae-in, who met Mr Kim only last month, is meeting security aides to discuss Mr Trump's announcement.
"[We] are trying to figure out what President Trump's intention is and the exact meaning of it," a government spokesman Kim Eui-kyeom said, in a comment carried on the Yonhap agency.
UN Secretary General Antonio Guterres said he was "deeply concerned" by the cancellation of the talks.
"I urge the parties to continue their dialogue to find a path to a peaceful and verifiable denuclearisation of the Korean peninsula," he added.
In the US, Republican Senator Tom Cotton praised President Trump for "seeing through Kim Jong-un's fraud". But Democratic Senator Brian Schatz said the move was what happened "when amateurs are combined with warmongers".