Washington said on Thursday that a meeting between presidents Donald Trump and Vladimir Putin would not be held until Moscow released three Ukrainian navy vessels and their crews, seized last month off Crimea.

“Of course, such a position cannot be grounds for violating the course of legal proceedings and the investigation that is under way against those who violated Russia’s state borders,” Kremlin spokesman Dmitry Peskov said on a conference call.

Russia detained the vessels’ combined crew of 24 last month and accused them of illegally entering Russian waters. Ukraine said Russia captured the two small gunboats and one tugboat illegally and accused Moscow of military aggression.

Putin and Trump were due to have an extended meeting at a G20 summit in Argentina two weeks ago, but Trump canceled after the naval incident.

Peskov said the Kremlin remained ready to organize a future meeting between Putin and Trump, as well as at other levels.

“We remain convinced that this meeting is equally necessary for Moscow as well as for Washington,” he said.