The US condemned Iran's seizing of two Greek-flagged oil tankers in the Arabian Gulf and demanded their release.
State Department spokesman Ned Price called the move a “threat to maritime security”.
US Secretary of State Antony Blinken and Greek Foreign Minister Nikos Dendias agreed that “Iran must immediately release the seized vessels, their cargoes and their crews”, Mr Price said.
“Iran's continued harassment of vessels and interference with navigational rights and freedoms are a threat to maritime security and the global economy,” he said.
“The United States stands with Greece, our key Nato ally and partner in the face of this unjustified seizure.”
Iran's Islamic Revolutionary Guard Corps boarded the Greek tankers in the Gulf on Friday, days after Athens said it would deliver Iranian oil it had seized from a Russian tanker to Washington.
Iran said on Saturday the crews of the tankers were in “good health” and not under arrest.
The crews have remained on board the two tankers. Greece said nine of its citizens are among the ship workers, but has not specified the number of other sailors on board.
Greece called Tehran's behaviour “tantamount to acts of piracy” and told citizens not to travel to Iran.
The IRGC, the ideological arm of Iran's military, said it took the tankers “due to violations”.
Greece said one of the tankers was in international waters, while the second was close to the Iranian coast when it was boarded.
Iranian navy helicopters dropped armed men on to the tankers, Greek officials said.
