SEOUL, May 25 (Xinhua) -- South Korean President Moon Jae-in expressed his deep regret over the cancelled summit between the Democratic People's Republic of Korea (DPRK) and the United States, Moon's office said early on Friday.

"It is embarrassing and very regrettable for the scheduled North Korea (DPRK)-U.S. summit not to be held on June 12," Moon said at the urgent meeting of the National Security Council (NSC).

Moon convened the meeting for an hour from midnight, right after U.S. President Donald Trump cancelled the first-ever summit between Pyongyang and Washington originally scheduled for June 12 in Singapore.

Moon said the Korean Peninsula's denuclearization and building permanent peace are historic tasks that cannot be delayed nor be given up.

Moon's expression of deep regret came as Trump said in a letter to the DPRK that "it is inappropriate" to hold the planned DPRK-U.S. summit on June 12 in Singapore.

Trump's cancellation of the DPRK-U.S. summit came after Pyongyang blew up tunnels at its Punggye-ri underground nuclear test site, where all of its six nuclear tests were conducted, earlier in the day.

The DPRK invited journalists from China, Russia, the United States, Britain and South Korea to let them cover the dismantling of the nuclear test site.

Kim Eui-kyeom, spokesman for the South Korean president, told reporters that the Blue House was attempting to figure out the exact meaning of Trump's cancellation, according to local media reports.