North Korea said the sentence was appropriate and called speculation that he died as a result of beating or torture "groundless".
Thousands of family members, friends and supporters of Otto Warmbier have gathered in Cincinnati to say goodbye to the American student who died days after returning to the United States in a coma following 17 months in captivity in North Korea.
President Trump called Warmbier's death "a total disgrace" on Tuesday, and he also tweeted that China's efforts to help with North Korea have "not worked out".
Warmbier was freed after the U.S. State Department's special envoy on North Korea, Joseph Yun, traveled to Pyongyang and demanded the student's release on humanitarian grounds, capping a flurry of diplomatic contacts, a U.S. official has said.
KCNA says the North dealt with Warmbier according to domestic law and worldwide standards.
"Why the USA government which claims to care about the welfare of its citizens had not even once made an official request for the release of Warmbier on humanitarian basis during the Obama administration?" the statement said.
Some 2,500 mourners lined up early on Thursday morning at Wyoming High School, in the Cincinnati suburb of Wyoming.
U.S. doctors who travelled to the North last week to bring Mr Warmbier home recognised that the North had provided him with medical treatment, the spokesman said.
The North Koreans said that constituted a "hostile act" - and sentenced him to 15 years of hard labor.
Charlie Dent 'concerned' by Senate health care bill
The bill would replace its individual insurance coverage with new subsidies and requirements and cut federal funding for Medicaid. The Senate is scheduled to vote Thursday on the Republican-backed bill .
Defectors and analysts have claimed abuse like that inflicted upon USA student Otto Warmbier is a routine experience for those in North Korea's concentration camps.
USA doctors said he had suffered a severe neurological injury from an unknown cause.
Warmbier's parents said they believe their son was subjected to "awful, torturous mistreatment" by the North Korean regime.
The spokesman said it was a "mystery" as to why Warmbier died days after returning home, but compared his death to the case of another American detainee, Evan Hunziker.
What does the North say to accusations it caused Mr Warmbier's death?
In a statement issued on Friday, North Korea claimed that the sudden death of Otto Warmbier was a mystery even to their nation as his health indicators were normal, as reported by Yonhap News.
American doctors who treated Warmbier claimed to find no evidence of botulism. No autopsy was performed, at the request of his family.
Mattis was speaking alongside Secretary of State Rex Tillerson after the two met Chinese officials to discuss North Korea and other regional issues, including China's continued military build up in the South China Sea.
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