The North's foreign ministry spokesman also said in comments carried by the official KCNA agency that Warmbier was "a victim of the policy of strategic patience" of former US President Barack Obama whose government never requested his release.
"The fact that Warmbier died suddenly in less than a week just after his return to the US in his normal state of health indicators is a mystery to us as well," the spokesman was quoted by KCNA as saying.
Warmbier, 22, was arrested in the reclusive country while visiting as a tourist. He was sentenced to 15 years of hard labour for trying to steal an item bearing a propaganda slogan from his hotel, North Korea state media said.
He was brought back to the United States last week in a coma with brain damage, in what doctors described as state of "unresponsive wakefulness", and died on Monday.
His death heightened the conflict between the North and the United States already aggravated by North Korea's defiant missile launches and two nuclear tests since early last year as part of its effort to build a nuclear-tipped intercontinental ballistic missile (ICBM) capable of hitting the US mainland.
US President Donald Trump blamed "the brutality of the North Korean regime" for Warmbier's death and South Korean President Moon Jae-in, who had advocated dialogue with the North, said Pyongyang had a "heavy responsibility" in the events leading up to the American's death.
The North's spokesman said such accusations are part of a smear campaign to slander the country that had given "medical treatments and care with all sincerity" to a person who was "clearly a criminal".
US doctors who had travelled to the North last week to evacuate Warmbier had recognized that the North had "provided him with medical treatment and brought him back alive whose heart was nearly stopped," the unnamed ministry spokesman said.
"Although Warmbier was a criminal who committed a hostile act against the DPRK, we accepted the repeated requests of the present US administration and, in consideration of his bad health, sent him back home on humanitarian grounds," the spokesman said.
DPRK is short for the North's official name, the Democratic People's Republic of Korea.
The exact cause of Warmbier's death remains unclear. Officials at the University of Cincinnati Medical Center, where he was treated after his return from the North, declined to provide details, and his family asked the Hamilton County Coroner on Tuesday not to perform an autopsy.
Thousands of friends and family members gathered at Wyoming High School in suburban Cincinnati on Thursday for a memorial service for Warmbier, who graduated from the school as salutatorian in 2013.
The United States has demanded North Korea release three other US citizens it holds in detention: missionary Kim Dong Chul and academics Tony Kim and Kim Hak Song.
Warmbier was freed after the US State Department's special envoy on North Korea, Joseph Yun, travelled to Pyongyang and demanded the student's release on humanitarian grounds, capping a flurry of diplomatic contacts, a US official has said.
The North previously released American detainees it had accused and convicted of crimes against the state on the occasion of high-level visits by US officials.