The U.K. has imposed sanctions on 18 more Russian oil tankers and four liquified natural gas (LNG) vessels, the largest batch of sanctions action to date against the country's so-called "shadow fleet," the government said Thursday.

The 18 oil tankers will be barred from U.K. ports and unable to access British maritime services, bringing the total number of sanctioned Russian oil tankers to 43.

"The U.K.'s relentless action against the shadow fleet is putting grit into the system and starving Putin's war machine of crucial revenues," the government said, adding the targeted oil tankers have transported an estimated 4.9 billion pounds ($6.37 billion) worth of product last year.

The U.K. also sanctioned Russian gas company Rusgazdobycha JSC.

The government added that its Department for Transport was working with maritime authorities to demand that Russian vessels with suspected dubious insurance provide details of their insurance status as they pass through the English Channel.

"Any actor that facilitates and supports Russia's malign activities could be exposing themselves to sanctions," the government said.

The Russian Embassy in London did not immediately respond to a Reuters request for comment on the sanctions.