US President Donald Trump has downplayed the prospect of achieving a diplomatic breakthrough with North Korea hours after Pyongyang launched its second suspected missile launch in less than a week.

"I don't think they're ready to negotiate," CNN quoted Trump as saying from the White House on Thursday.

"We're looking at it very seriously right now," the President said of the latest North Korean launch. "Nobody's happy about it."

Earlier on Thursday, North Korea launched two suspected "short-range missiles", the South Korean military confirmed.

The latest provocation comes after North Korea tested several new weapons systems late last week. They were the first confirmed launches of their kind since 2017.

Despite the setbacks, Trump said on Thursday that "the relationship continues".

Just prior to the President's remarks, the US Justice Department announced that it had seized a North Korean cargo ship, alleging sanctions violations by the country.

The ship, the M/V Wise Honest, was "used to illicitly ship coal from North Korea and to deliver heavy machinery to (North Korea)", the Department said, alleging that "payments for maintenance, equipment, and improvements of the Wise Honest were made in US dollars through unwitting US banks".

"This conduct violates longstanding US law and United Nations Security Council resolutions," it added.

Thursday's developments are the latest sign of deterioration in the fragile negotiations between the US and North Korea following the failed summit between Trump and Pyongyang's Kim Jong-un in February.