Russia said on Wednesday that the US seizure of a Russian-flagged oil tanker in the Atlantic was a violation of maritime law.
"In accordance with the 1982 UN Convention on the Law of the Sea, freedom of navigation applies in the high seas, and no state has the right to use force against vessels duly registered in the jurisdictions of other states," the ministry said in a statement.
It said contact with the vessel, the Marinera, had been lost after US naval forces boarded it.
A senior Russian lawmaker from the ruling United Russia party, Andrei Klishas, said the seizure was an act of outright piracy, the TASS state news agency reported.
The US said on Wednesday it had seized a Russian-flagged, Venezuela-linked tanker as part of President Donald Trump's aggressive push to dictate oil flows in the Americas and force Caracas' socialist government to become its ally.
Having deposed Venezuelan leader Nicolas Maduro, Washington is blockading sanctioned vessels in and out of waters off the South American OPEC member.
The US Coast Guard and military apprehended the Marinera tanker that refused to be boarded last month and switched to Moscow's flag, officials said.
With a Russian submarine and vessels nearby after a two-week pursuit in the Atlantic, the move risked more confrontation with Moscow which has condemned US actions over Venezuela and is already at odds with the West due to the Ukraine war.
Russian state broadcaster RT showed an image of a helicopter hovering near the Marinera, originally known as the Bella-1, and said it looked like U.S. forces were trying to board the US-sanctioned tanker, which is empty.
The US Coast Guard also intercepted another fully-loaded Venezuela-linked tanker, Sophia, near the northeast coast of South America, the officials said, in the fourth such case of recent weeks.
