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.
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"[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.