Greenpeace protesters have blocked the entry of a Greek tanker into a southern English port due to its Russian fuel cargo with police making arrests, the green group said on Monday, drawing condemnation from the British government.

Britain and the European Union have separately banned Russian-flagged vessels from their ports, with exemptions, following Russia's invasion of Ukraine which the Kremlin describes as a "special military operation".

The United Kingdom has said it will phase out imports of Russian oil by the end of the year, which has meant that foreign flagged vessels carrying Russian cargoes are still able to call at ports for the time being.

Greenpeace said 12 activists occupied a jetty on Sunday evening in Navigator Terminals' Thames site in Essex, where the Greek-flagged Andromeda oil products tanker was expected to discharge its cargo.

"Ministers have kicked a ban on Russian oil imports to the end of the year despite strong public support for it," said Georgia Whitaker, oil and gas campaigner with Greenpeace UK.

A British government spokesperson said: "There is absolutely no excuse for Greenpeace's disruptive actions, which are wholly unacceptable."

Essex Police said on Monday that the site had now been cleared after officers were called to the Navigators Terminal late on Sunday evening. As of 1240 GMT they had made a total of 15 arrests, all of whom remained in custody.

"Policing is not anti-protest, but we must intervene where there is a risk to life or where there is a suspicion laws are being broken," Essex Police said.

Greenpeace said the 15 activists arrested included some of their team who were on a boat near the jetty.

A spokesperson for Navigator Terminals referred comment to Essex Police.

The tanker's Athens-based manager, Tsakos Columbia Shipmanagement, did not immediately respond to a request for comment.

In recent weeks, a number of ships carrying Russian cargoes have changed course after protests at both UK and EU ports.

Refinitiv ship tracking data showed the Andromeda had previously called at the Russian port of Primorsk and was almost fully laden. The vessel was anchored off the Essex coast on Monday and reported the Isle of Grain in nearby Kent as its next destination, ship tracking data showed.