The US has imposed sanctions on a dozen Russian and Chinese companies and individuals it accuses of helping North Korea’s nuclear weapons programme.
It comes after the UN Security Council, including Russia and China, voted for further sanctions against Pyongyang.
The US Treasury said its actions would “increase pressure” on North Korea, but the move has angered China.
US Secretary of State Rex Tillerson, meanwhile, praised North Korea for “a level of restraint” in recent days.
“We have had no missile launches or provocative acts on the part of North Korea since the unanimous adoption of the UN Security Council resolution,” he said.
This, he said, could pave the way for talks between the two sides “sometime in the near future”.
Isolating North Korea
The US Office of Foreign Assets Control designated 10 companies and six individuals in its sanctions.
“[The] Treasury will continue to increase pressure on North Korea by targeting those who support the advancement of nuclear and ballistic missile programmes, and isolating them from the American financial system,” said Treasury Secretary Steven Mnuchin.
The action means American individuals and companies are no longer permitted to do business with these firms.
China responded swiftly, calling on the US to “immediately correct its mistake” of punishing its firms.
A series of missile tests by North Korea in recent months – along with its repeated threats to carry out a sixth test of a nuclear device – have increased tensions between Pyongyang and the US.
North Korea has been angered, as it is every year, by scheduled US-South Korea military drills, and threatened to launch missiles near the US island of Guam in the South Pacific.
US President Donald Trump, meanwhile, threatened the isolated regime with “fire and fury like the world has never seen”.
In North Korea’s latest propaganda video, released on Tuesday, an image of Mr Trump is shown at a cemetery which is apparently meant to be in Guam.
Vice-president Mike Pence is also pictured engulfed in flames.
Mr Tillerson’s comments on Tuesday appeared to strike a more conciliatory tone.
He said North Korea had not launched any missiles since the UN unilaterally imposed new sanctions, and had “demonstrated some level of restraint that we’ve not seen in the past”.
“We hope that this is the beginning of this signal that we’ve been looking for – that they are ready to restrain their level of tensions, they’re ready to restrain their provocative acts and that perhaps we are seeing our pathway to sometime in the near future having some dialogue.”
However, speaking at UN-backed disarmament conference in Geneva on Tuesday, a North Korean diplomat insisted that the weapons programme was “justifiable and a legitimate option for self-defence”.
“As long as the U.S. hostile policy and nuclear threat remains unchallenged, the DPRK will never place its self-defensive nuclear deterrence on the negotiating table or step back an inch from the path it took to bolster the national nuclear force,” he said.