WASHINGTON – President Trump said Monday he plans to declare Kim Jong Un's government a state sponsor of terrorism, as he seeks to ratchet up the pressure on North Korea over its nuclear weapons program.

"The North Korean regime must be lawful. It must end its unlawful nuclear and ballistic missile development, and cease all support for international terrorism – which it is not doing," Trump said during a Cabinet meeting at the White House.

In decrying Kim's "murderous regime," Trump cited the recent death of American citizen Otto Warmbier after he was taken into custody in North Korea. He also appeared to reference North Korea's support of an assassination of Kim's half-brother Kim Jong Nam, a political rival who sustained a nerve agent attack at a Malaysia airport in February.

“In addition to threatening the world by nuclear devastation, North Korea has repeatedly supported acts of international terrorism including assassinations on foreign soil,” Trump said.

Trump made the announcement one week after returning from a trip to Asia, where he asked China and other countries in the region to tighten the financial and diplomatic vise on North Korea.

As part of that effort, Trump said his Treasury Department on Tuesday will announce an additional economic sanction on North Korea, and "it will be the highest level of sanctions by the time it's finished over a two-week period."

Trump said the terror declaration "should have happened a long time ago." Yet the United States has previously declared North Korea to be a state sponsor of terrorism – for two decades, in fact, from 1988 to 2008.

The George W. Bush administration took the country off the list in 2008 as part of an ultimately failed agreement with North Korea to curb its nuclear weapons program.

Secretary of State Rex Tillerson said the Trump administration still hopes that diplomatic efforts will be successful in persuading North Korea to abandon its nuclear weapons program.

In the meantime, Tillerson said, the United States will continue to impose sanctions, and "continue to turn the pressure up on North Korea by getting other countries to join and take actions on their own."

Last month, 12 senators — six Republicans and six Democrats — urged Tillerson to put North Korea back on the list of countries that the U.S. considers sponsors of terrorism. North Korea would join Iran, Sudan and Syria on the U.S. list of state sponsors of terror.

Designating the rogue nation as a sponsor of terror would give the U.S. another pressure point against North Korea. It would also be a “diplomatic setback” for the country, Marc Thiessen, a former Bush aide and foreign policy expert, wrote in a recent op-ed arguing in favor of the move.

“The additional sanctions that come with a re-designation may not make much of a difference in North Korean’s behavior, but they are one piece of a larger strategy for isolating and squeezing the North Korean regime,” Thiessen wrote.

Trump and other supporters of the new listing have said the case of Warmbier, an American student who died of injuries sustained while in custody in North Korea, was part of the impetus for the decision. Kim's government had sentenced Warmbier to 15 years hard labor over claimed he tried to steal a poster from a staff-only section of a hotel in North Korea.

Seattle expert agrees with administration’s decision

“They deserve the designation,” said Dr. Richard Ellings, President of the National Bureau of Asian Research.

“(Kim Jong Un) is going to have to live with it. I suspect you’ll hear something at least verbally back and forth. You might have a missile test. I don’t know, but it’s something we had to do.”

North Korea was removed from the list of state sponsors of terrorism under President George W. Bush in 2008, as part of an effort to warm up diplomatic relations and potentially strike a nuclear deal. However, ultimately the negotiation failed and North Korea continued working on his nuclear program.

“We’ve now wisened up. We understand they have consistent, enduring interests; that nuclear weapons serve those interests. They don’t serve ours, but they serve North Koreas. We have to be consistent in our policy.”

While the move is largely symbolic, Ellings says it sends an important message to not only North Korea, but other key players in the region.

“What it does is try to underscore our commitment to being as firm as we can be and communicating with China that we’re not making any compromises me. China is the one power that can make a difference, in my view, this communicates to them that we’re not softening our stand,” explained Ellings.

While diplomatic leaders continue to work on a strategy to encourage China to increase pressure on North Korea, a key trade partner, Ellings says he is encouraged by an extremely strong relationship between the U.S. and ally South Korea.

“It’s as solid as it can be,” said Ellings who just returned from the region.

His trip overlapped with that of the Trump administration earlier this month, and he says he was left with increased confidence in the overall U.S. strategy.

“The policy seems sensible, strong and stabilizing and clear,” said Ellings. “Our deterrence is greater today than it was a month or two or three ago. That’s really key, but big decisions lie ahead about how we sustain that deterrence.”