The news quickly sparked calls from Congress for a tougher line. "The United States of America cannot and should not tolerate the murder of its citizens by hostile powers," Republican Sen. John McCain of Arizona said in a statement.

"North Korea is threatening its neighbors, destabilizing the Asia-Pacific region, and rapidly developing the technology to strike the American homeland with nuclear weapons," McCain continued. "Now it has escalated to brutalizing Americans, including three other citizens currently imprisoned in North Korea."

Warmbier's death comes two days before the annual US-China diplomatic and security dialogue in Washington where Secretary of State Rex Tillerson and Defense Secretary James Mattis will meet their Chinese counterparts.

Pressure on Trump

The young Ohioan's death may force President Donald Trump to take a tougher line with North Korea, a shift that could increase tensions with Beijing. "Like Obama, Trump has been talking tough, but continuing to pull punches on both North Korea and China," Klingner said.

Klingner pointed in particular to the Trump administration's failure so far to apply "secondary sanctions" on Chinese companies that do business with North Korea, particularly the companies involved in the country's development of nuclear and ballistic missile technology.

The administration has imposed penalties on a "limited tranche of North Korea violators and continues to not impose secondary sanctions on Chinese violators given a naïve belief that President Xi will do more than China has in the past," Klingner said. "We've heard that song many times before and always been disappointed."

Leon Panetta, who served as Secretary of Defense under President Obama between 2011 and 2013, noted that "we have sanctions in place," adding that "they could be enforced in a more vigorous way by ourselves as well as the Chinese."

Panetta said that in the wake of Warmbier's death, the Trump administration's options include demanding an explanation from China, diplomatic protests, and increased sanctions. "This is not acceptable and the Chinese government needs to find out what happened," Panetta told CNN's Wolf Blitzer.

US reactions

On Monday, the President expressed dismay at the news of Warmbier's death, calling North Korea a "brutal regime" and telling tech CEOs gathered at the White House that a lot of bad things happened to the student while he was held there.

In a written statement, Trump said, "Melania and I offer our deepest condolences to the family of Otto Warmbier on his untimely passing. There is nothing more tragic for a parent than to lose a child in the prime of life. Our thoughts and prayers are with Otto's family and friends, and all who loved him."

"Otto's fate deepens my administration's determination to prevent such tragedies from befalling innocent people at the hands of regimes that do not respect the rule of law or basic human decency," Trump's statement continued. "The United States once again condemns the brutality of the North Korean regime as we mourn its latest victim."

US Ambassador to the UN, Nikki Haley, also issued a statement that said Warmbier's fate would be "an indelible reminder to us of the barbaric nature of the North Korean dictatorship."

Tillerson said that "we hold North Korea accountable for Otto Warmbier's unjust imprisonment and demand the release of three other Americans who have been illegally detained."

North Korea will be a top priority for Tillerson and Mattis going into Wednesday's talks with China, said Susan Thornton, the State Department's acting assistant secretary for the bureau of East Asian and Pacific Affairs.

Thornton told reporters Monday that US officials are "aiming to advance concrete cooperation with China towards a peaceful resolution of the nuclear and missile threat from North Korea." Noting that repeated UN resolutions forbid the DPRK's ballistic missile and nuclear programs, Thornton added that, "we continue to urge China to exert its unique leverage as North Korea's largest trading partner, including by fully implementing all UN Security Council sanctions."

Klingner said that during the US meetings with China, "the message should be very clear that the US agreement to hold back on secondary sanctions should be coming to a close."

He added that "the Trump administration's hesitancy to impose secondary sanctions, like that of previous administrations, is simply giving de facto immunity from US law to Chinese violators."

Nuclear ambitions

Klingner pointed to recent declarations by North Koreans in unofficial meetings with US figures -- sometimes referred to as "track two" or "track 1.5" dialogues. At one such meeting recently, North Korean officials stated that denuclearization is off the table and that there's nothing the US or South Korea can offer that will induce them to stop pursuing nuclear capability.

After multiple missile launches in May, the UN Security Council voted unanimously on June 2 to introduce new sanctions targeting Pyongyang's nuclear and missile program. Five days later, North Korea responded by firing four anti-ship missiles into the sea east of the Korean peninsula.

Administration officials have studiously avoided talking about retaliation for the condition in which Warmbier's condition, most recently Monday morning, before Warmbier's death, when Thornton told reporters that "we are very happy to see that Otto Warmbier has been able to return to his family in Ohio and be reunited with them."

The former University of Virginia student was comatose when he returned early from a 15-year prison sentence of hard labor for stealing a political poster. North Korea said his medical state was the result of botulism poisoning.

Intelligence reports suggest he had been repeatedly beaten.