Washington DC - The top Senate Democrat on military affairs said Monday President Donald Trump's order to scale back drills with ally South Korea was "inane" and "haphazard."

Trump said he ordered the Pentagon to downsize "inappropriate and hostile" drills, hours before the exercises were due to begin on Monday, citing his "good relationship" with North Korean leader Kim Jong Un.

In a post Sunday on Truth Social, Trump also cited South Korea's refusal to join the US in the "denuclearization" of Iran.

"This is another inane, haphazard decision by President Trump and his chaotic administration," Senate Armed Services Committee ranking member Jack Reed said in a statement.

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"When President Trump cuts military exercises to flatter a dictator or punishes South Korea for declining to join a war he started, all of our allies – and adversaries – think that America's commitments are negotiable."

He called the decision a "propaganda victory" for North Korea's Kim and punishment "for no reason beyond Trump's ego" for South Korea.

"China and Russia are watching how easily this president abandons America's allies, and they will remember it the next time they test us," Reed said.

Washington and Seoul have been conducting joint military exercises together since the 1950s as a deterrence to what Reed called North Korean aggression, and as a signal of strength to other potential adversaries.

Reed said Trump's post contradicted a recent warning from NORTHCOM Deputy Commander Lt. Gen. Joseph Jarrard that Pyongyang had successfully flight-tested intercontinental ballistic missiles with "enough boost to deliver a nuclear payload anywhere in North America."

North Korea has not responded to Trump's order, though it has previously voiced anger at the regular joint maneuvers, which take place in South Korea.

The drills began as American allies in East Asia fret over North Korea's displays of weapons capabilities and its growing alignment with Russia as well as traditional backer China.