US Senator John McCain says North Korea has murdered Otto Warmbier, an American student who died days after his release from detention there.
McCain, the Republican chairman of the Armed Services Committee, condemned the treatment of the 22-year-old University of Virginia student, who he said was "murdered by the Kim Jong-un regime".
"The United States of America cannot and should not tolerate the murder of its citizens by hostile powers," McCain said in a statement on Monday.
Warmbier was medically evacuated from Pyongyang to the US last week, after having suffered extensive brain damage that left him in a state of "unresponsive wakefulness". His parents said he died on Monday.
His death comes amid high tension over dozens of North Korean missile launches and two nuclear bomb tests, and has spurred renewed calls for action against the isolated country.
US President Donald Trump, in a statement offering condolences to Warmbier's family, said the student's fate deepened his determination to prevent other such tragedies.
"It's [North Korea] a brutal regime, and we'll be able to handle it," he told reporters later.
Warmbier was arrested at the airport as he was leaving Pyongyang in January last year and sentenced to 15 years of hard labour at a show trial for stealing a political poster from a hotel.
The South Korean government meanwhile pledged to make every effort for the return of three other US citizens, who are ethnic Koreans, and six South Koreans, who remain in custody in North Korea.
South Korean President Moon Jae-in also urged the North to swiftly return the nine detainees, and said it was "deplorable that North Korea does not respect human rights", according to a presidential spokesman.
Young Pioneer Tours, the China-based travel agency that organised Warmbier’s trip to North Korea, said on Tuesday it would no longer take US citizens to the country.
The agency said his death shows that the risk Amercian tourists face in visiting North Korea "has become too high".
The North claimed Warmbier fell into a coma soon after he was sentenced last year, saying he had contracted botulism and been given a sleeping pill.
However, medical tests carried out last week in the US offered no conclusive evidence as to the cause of his neurological injuries, and no evidence of a prior botulism infection.
Warmbier's doctors said he had suffered extensive tissue loss in all regions of his brain, but showed no signs of physical trauma.
They said Warmbier's severe brain injury was most likely - given his young age - to have been caused by cardiopulmonary arrest cutting the blood supply to the brain.
US rights group Human Rights Watch said Warmbier's death highlights the North's position as "one of the worst rights-abusing governments in the world."
His death "reflects a reality that the North Korean people know all too well: the Kim family leadership ... will not hesitate to brutalise and kill to maintain their hold on power", it said in a statement.
Source: News agencies