PYONGYANG - The Democratic People's Republic of Korea (DPRK) said Friday the death of American student Otto Warmbier days after his release was a "mystery," as his health indicators and examination results were all normal when he left DPRK.

A spokesman the country's foreign ministry said the DPRK gave Warmbier all available medical treatment and care on a humanitarian basis until his return to the United States, and that Warmbier was not mistreated or tortured during his 18-month detention in DPRK.

"Although we had no reason at all to show mercy to such a criminal of the enemy state, we provided him with medical treatment and care with all sincerity on a humanitarian basis until his return to the US," the Korean Central News Agency quoted the spokesman as saying.

Warmbier, 22, was released on June 13 and died on June 19 after arriving home in a coma, according to earlier foreign media reports.

He was sentenced by DPRK court to 15 years of hard labor in March last year for anti-DPRK activities at the order of an American religious group and the Central Intelligence Agency during stay in Pyongyang. Warmbier claimed to be a tourist.

"As for groundless public opinion now circulating in the US that he died of torture and beating during his reform through labor, the American doctors who came to the DPRK for repatriation of Warmbier will have something to say about it," said the DPRK spokesman.

The US doctors "examined Warmbier and exchanged medical observations about him with" DPRK doctors, and "recognized that his health indicators like his pulse, temperature, respiration and the examination results of the heart and lung were all normal," said the spokesman.

"We provided him with medical treatment and brought him back to life when his heart nearly stopped" beating, the spokesman added.