Vladimir Putin has escalated Russia’s shadow war at sea by putting armed guards on board sanction-busting oil tankers, warn Western officials.

Swedish naval commanders report seeing uniformed, armed personnel on “shadow fleet” vessels in the Baltic Sea, backed by Russian warships patrolling nearby shipping lanes.

The move underlines how vital the illicit fleet is to funding Moscow’s war, with the Kremlin now prepared to commit military assets to protect oil exports in defiance of Western sanctions.

For now it is unclear who the armed personnel are but it is believed they may be from “private security companies” deployed by the Russian authorities.

While the trend has been spotted in the Baltic, many of the same vessels sail through the North Sea and English Channel.

“We have seen and received information that there are people in uniform on board some of these shadow fleet vessels,” said Marko Petkovic, head of the Swedish Navy’s operational command.

Russia has been increasing its military surveillance and presence in the Baltic Sea for some time, he said.

“The Russian navy is periodically present at various key points in the Baltic Sea, the Gulf of Finland….and appears to be supporting this shadow fleet in some way.”

Daniel Stening, deputy chief of operations for the Swedish Coastguard, said the military presence showed the strategic importance of the shadow fleet for Moscow.

“We know that this activity is very important for Russia, so we assume that various security measures have been taken, and that such measures could well be in place,” he said.

Petkovic said the Russian naval presence had “become more permanent and present in large parts of the Baltic Sea”.