The United States took over the Russian-flagged Marinera earlier this month, alleging it was part of a shadow fleet of tankers carrying oil to countries such as Venezuela, Russia and Iran in violation of US sanctions.

"We were assured that a decision has been made at the highest level to secure their release," Russian Foreign Minister Sergei Lavrov told a news conference.

"Unfortunately, the following days showed that this decision is not being implemented. We expect our American colleagues to fulfil their promises that were made to us," he added.

The United States said publicly that the Marinera's crew could be prosecuted -- which Russia said was "categorically unacceptable."

The United States seized the tanker, which was being escorted by the Russian navy, after chasing it from near the Venezuelan coast.

It was re-flagged and re-named to bring it under Russian jurisdiction in a bid to discourage the United States from trying to take it as part of its campaign against Venezuela.

Moscow had accused Washington of stoking tensions and threatening international shipping, casting the seizure as illegal.