By Lucy Papachristou and Mark Trevelyan

Jan 7 (Reuters) - Russia said on Wednesday that the U.S. seizure of a Russian-flagged oil tanker in the Atlantic was a violation of maritime law, and a senior lawmaker described it as "outright piracy".

Russia's Transport Ministry said contact with the vessel, the Marinera, had been lost after U.S. naval forces boarded it near Iceland as part of efforts to block oil exports from Venezuela.

"In accordance with the 1982 U.N. 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.

Russia is demanding that the United States ensure humane and decent treatment of the Russian crew members and their swift return home, state news agency TASS quoted the Foreign Ministry as saying.

The Marinera, originally known as the Bella-1, had previously slipped through a U.S. maritime blockade of sanctioned tankers in the Caribbean.

U.S. WAGING PRESSURE CAMPAIGN AGAINST VENEZUELA

The blockade was part of a U.S. pressure campaign against Venezuela that culminated when President Donald Trump sent in U.S. special forces on January 3 to capture Venezuelan President Nicolas Maduro and bring him to New York to face drug trafficking charges, which he has denied.

"After a 'law enforcement operation' that killed several dozen people in Venezuela, the U.S. has engaged in outright piracy on the high seas," Andrei Klishas, a lawmaker from the ruling United Russia party, posted on Telegram.

Two U.S. officials, speaking on condition of anonymity, told Reuters that Wednesday's operation was carried out by the Coast Guard and U.S. military.

They said Russian military vessels, including a submarine, were in the general vicinity. There were no indications of any confrontation between U.S. and Russian military forces.

Relations between Moscow and Washington plunged to their worst state since the Cold War after Russia invaded Ukraine in February 2022, though they have become more cordial since Trump began his second term last year and started engaging with President Vladimir Putin to seek an end to the conflict.

Military incidents between the nuclear-armed powers are rare. In March 2023, a U.S military surveillance drone crashed into the Black Sea after being intercepted by Russian fighter jets, prompting Washington to protest and warn of the risk of an escalation.

Venezuelan leader Maduro was the second close ally of Russia to be ousted in just over a year, following the toppling of Syrian President Bashar al-Assad in December 2024.

Russia has said it backs Delcy Rodriguez, sworn in on Monday as interim president, and will continue to support Venezuela in the face of what Moscow has called "blatant neocolonial threats and foreign armed aggression".

With Russians still in the midst of an extended New Year holiday period, Putin has yet to comment publicly on the U.S. action to remove Maduro.

(Reporting by Lucy Papachristou in Tbilisi and Mark Trevelyan in London; additional reporting by Andrew OsbornEditing by Gareth Jones)