The State Department has called the ratification of a contested maritime pact between Turkey and Libya "provocative."
It is Washington's bluntest criticism yet of a deal that has inflamed tensions in the eastern Mediterranean.
"The announcement of the ratification of the Turkey-Libya memorandum of understanding on maritime delimitation has increased tensions in the region, is not constructive and is provocative," the department said in a statement.
"Our position on this issue has not changed."
The language marks a notable shift.
Washington has long tried to stay neutral in maritime quarrels between Greece and Turkey, both NATO allies.
Just four days earlier, its first response to the ratification had avoided criticism, simply urging negotiations and pointing to the economic rewards of a settlement. Secretary of State Marco Rubio, during a visit to Athens this month, also spoke of the region's "sensitivities," a nod to Ankara.
Sunday's statement followed a letter from the Hellenic American Leadership Council, an advocacy group, urging Mr. Rubio to reject the deal publicly.
The 2019 memorandum, signed by Turkey and Libya's government in Tripoli, carves out a maritime boundary between the two countries that ignores Greek islands, including Crete. Libya's rival parliament in the east, which had blocked the deal for years, ratified it last week. Greece, Cyprus, Egypt and the European Union have called it illegal.
The dispute is more than a quarrel over lines on a map.
The waters at stake hold potential gas reserves and lie along the route of a planned undersea power cable linking Greece and Cyprus, a project Turkey opposes. A French survey ship is due in Crete on Tuesday to resume work on the cable. In 2024, Turkish warships shadowed a similar ship and halted the work.
By calling the deal provocative, Washington signals that Ankara's move will carry diplomatic costs.
That could strengthen Athens' hand as the survey begins.
The State Department still urged Greece and Turkey to settle their differences through direct talks.