U.S. President Donald Trump announced Monday the U.S. is putting North Korea's "murderous regime" on America's terrorism blacklist, despite questions about Pyongyang's support for international attacks beyond the assassination of its leader's half brother in February.

Trump said the designation of North Korea as a state sponsor of terrorism was long overdue, and he promised a new wave of sanctions as part of a "maximum pressure campaign" over the North's development of nuclear weapons that could soon pose a direct threat to the U.S. mainland.

North Korea will join Iran, Sudan and Syria on the blacklist. The North had been designated for two decades until 2008 when it was removed in a bid to salvage international talks aimed at halting its nuclear efforts. The talks collapsed soon after and haven't been revived since.

The primary impact of the designation may be to compound North Korea's growing international isolation as it is already subject to an array of tough U.S. sanctions restricting trade, foreign assistance, defence sales and exports of sensitive technology. The step is likely to further sour relations between Washington and Pyongyang that have turned uglier with name-calling between Trump and Kim Jong-un.

Why would Kim Jong-un insult me by calling me "old," when I would NEVER call him "short and fat?" Oh well, I try so hard to be his friend - and maybe someday that will happen! —@realDonaldTrump

There is strong bipartisan support for the move in Congress, which had passed legislation in August requiring the State Department to make a determination on putting North Korea back on the list.

"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 as he announced the designation at a cabinet meeting at the White House.

However, the action had been debated for months inside the administration, with some officials at the State Department arguing that North Korea did not meet the legal standard to be relisted as a state sponsor of terrorism.

North Korean leader Kim Jong-un reacts during the test launch of ground-to-ground ballistic rocket, in an undated photo released by North Korea's Korean Central News Agency in June 2016. (KCNA/Reuters)

U.S. officials involved in the internal deliberations said there was no debate over whether the slaying of Kim's half brother Kim Jong-nam was a terrorist act. Malaysian authorities have said he was killed by two women who smeared suspected VX nerve agent onto his face at Kuala Lumpur airport Feb. 13.

However, lawyers said there had to be more than one incident, and there was disagreement over whether the treatment of American student Otto Warmbier, who died of injuries suffered in North Korean custody, constituted terrorism.

New sanctions expected

Neither Trump nor the State Department specified Monday which acts of terrorism and assassination the North had supported. In making the announcement, Trump did refer to Warmbier "and the countless others so brutally affected" by North Korean oppression.

He said more sanctions would be imposed on North Korea and "related persons" that the Treasury Department would begin to announce Tuesday — part of rolling effort to deprive Pyongyang of funds for its nuclear and missile programs and leave it internationally isolated.

"It will be the highest level of sanctions by the time it's finished over a two-week period," Trump said.

Secretary of State Rex Tillerson listens as Trump speaks during a cabinet meeting at the White House on Monday. (Evan Vucci/Associated Press)

Anthony Ruggiero, a sanctions expert at the Foundation for Defence of Democracies think-tank , said the designation does not grant sanctions authority that the administration does not already have but will help "push additional countries to cut commercial and diplomatic ties with North Korea."

The State Department said last week that Sudan, which is on the terror list itself, had agreed to cut all military and trade ties to North Korea. As the North has faced isolation from Western countries, it has increasingly sought relationships in Africa, the Middle East and South Asia in search of badly needed finances.

Evans Revere, a former senior State Department official, said North Korea is already livid with Trump and is likely to react "quickly and emotionally." A recent editorial in the ruling party Rodong Sinmum newspaper referred to the president as "a hideous criminal sentenced to death by the Korean people."

As talk of war on the Korean Peninsula heats up, U.S. President Donald Trump faces a tough choice: take action or wait and see if the rogue nation changes course 2:23

Other analysts said Pyongyang could use the designation as a pretext for renewed weapons tests after a two-month hiatus. The latest missile test overflew Japan Sept. 15.

Possible new sanctions steps could be to impose restrictions on Chinese banks that serve as North Korea's conduit to the international system. However, such a move would irk Beijing, whose help Trump is counting on to put an economic squeeze on Pyongyang.

North Korea was on the terrorism blacklist for two decades after the 1987 bombing of a South Korean airliner killed 115 people. It was also accused of a 1983 bombing assassination attempt against then-South Korean president Chun Doo-hwan in Myanmar. The president survived, but 21 others were killed. The North has not been publicly implicated in a terror attack of that scale since.

Link being looked into after hack attack hobbles studio behind comedy spoof 3:13

House legislation introduced this year had urged the State Department to review a list of purported acts by North Korea, including assassinations of dissidents and weapons sales to militant groups including Hamas and Hezbollah. It requested a determination as to whether such acts constitute support for international terrorism.

The legislation also cited the 2015 computer hack of Sony Pictures Entertainment, which the FBI blamed on North Korea. Hackers threatened movie theatres that screened The Interview, a comedy parodying the North's leader, Kim.

Canada does not identify North Korea as a state sponsor of terrorism. The only countries on Canada's list are Iran and Syria.