A file photograph of the PVT Azura, one of the vessels being tracked by Greenpeace and said to be carrying Russian diesel to the US.Photograph: Martin Witte/Alamy

Seven oil tankers carrying an estimated 288,000 tonnes of Russian diesel are now sailing towards the US, though it is uncertain if the vessels will be able to unload amid a political row about the legality of the shipments.

The transport, tracked by the pressure group Greenpeace, is the first phase of a contentious supply deal negotiated between presidents Donald Trump and Vladimir Putin earlier this month, reversing years of sanctions on Russia oil.