Iran was told on Thursday to come clean about undeclared nuclear material to end a stand-off with a UN watchdog.
A resolution passed by the International Atomic Energy Agency's 35-member board said it was "essential and urgent" for Iran to resolve the dispute.
It urges Iran to abide by its obligations under a non-proliferation agreement with the IAEA.
"Iran must now provide the necessary co-operation ― no more empty promises," said Laura Holgate, the US representative on the board.
The long-running impasse concerns uranium traces at three undeclared locations in Iran.
The IAEA has said it cannot be sure that Iran's nuclear activities are peaceful until it receives a satisfactory explanation.
The whereabouts of the uranium traces are unknown and they could still be in Iran, the EU delegation said in a statement to the board on Thursday.
"Iran has yet to take the necessary actions and provide technically credible explanations," it said at the closed-door meeting in Vienna.
Tehran has suggested it could not return to its 2015 nuclear deal with world powers until the IAEA investigation is out of the way.
But IAEA director general Rafael Grossi has continued to demand answers, and the US rejects any attempt to link the two matters.
Ms Holgate said the UN agency could take further action against Iran if it does not meet the board's demand.
"Iran continues to brazenly disregard the director general’s repeated requests ― for years now ― for Iran to provide the substantive co-operation necessary," she told the board.
"The power to resolve these issues is in Iran’s hands, and we once again urge Iran to do so. There are no shortcuts here: political or other."
Another US diplomat, special envoy for Iran Robert Malley, said this week that restoring the 2015 pact was not Washington's focus at present.
He said the US was more concerned with domestic repression in Iran and its alleged military support for Russia.
Russia, one of Iran's few allies on the IAEA board, cheered the fact that Thursday's resolution won slightly less support than another vote on Iran in June.
It came after the board was reconfigured in October, with several seats changing hands.