Outgoing U.S. Secretary of State John Kerry has warned that the North Korean nuclear issue will be among the most serious threats that the Donald Trump administration will have to deal with.

In an "exit memo" sent to other U.S. government officials on Thursday, Kerry said the North's nuclear program is “among the gravest" that the country faces today.

While criticizing the regime for its provocative and destabilizing actions, he emphasized that the U.S. does not accept the North as a nuke state and never will.

Noting Washington’s role in adopting the two strongest packages of sanctions on the North at the UN Security Council last year, Kerry assessed the Obama administration is handing over to the next U.S. administration "a strong foundation for increasing pressure on Pyongyang that will be critical to address this threat.”

Kerry urged the U.S. government to remain firm on its three-pronged campaign against the North focused on diplomacy, deterrence and pressure.

He said Washington should build a sustained, comprehensive and relentless campaign that increases the costs on North Korea until it makes a strategic decision to return to serious talks on denuclearization and complies with its international obligations.

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