Jailed Kremlin critic Alexei Navalny's anti-corruption group has vowed to fight on after a court branded it an "extremist" organisation and ordered its closure.

Western countries and the European Union were quick to condemn last night's ruling, but senior Russian officials doubled down, describing Navalny as an agent collaborating with Washington.

The court decision was the latest move against critics of President Vladimir Putin.

Some of his loudest opponents have fled the country and several prominent activist groups and independent media have closed.

The ruling bans Navalny's Anti-Corruption Foundation (FBK) and a network of regional offices from operating and, under a recently passed law, prevents those previously associated with the groups from running in parliamentary elections.

The FBK was defiant, saying in Twitter post: "We woke up, smiled with destructive intent and knowing that we are a 'danger to society' will continue to fight corruption!"

What exactly it will be able to do is unclear in the wake of the ruling, which followed a hearing behind closed doors.

Navalny's key allies still in Russia are under close law enforcement supervision, some under house arrest, and other prominent aides have gone into exile.

Navalny was jailed for more than two-and-a-half years in February after he returned from Germany where he had been convalescing following a poisoning attack in Siberia that he blamed on the Kremlin.

Russia's most prominent opposition leader, who is in a penal colony outside Moscow, acknowledged supporters would now have to alter their strategy.

"But we will not retreat from our goals and ideas. This is our country and we have no other," the 45-year-old said on Instagram.

The EU condemned the court ruling as the latest effort to "suppress" the opposition.

"It is an unfounded decision that confirms a negative pattern of a systematic crackdown on human rights and freedoms which are enshrined in the Russian constitution," EU foreign policy chief Josep Borrell said in a statement on behalf of the 27 member states.

UK Foreign Secretary Dominic Raab had described the ruling as "perverse" and "Kafka-esque," while the United States called on Moscow to end the crackdown and release Navalny.

US President Joe Biden has promised to raise the issue of human rights with Mr Putin when the two meet next week for a summit in Geneva.

The spokeswoman for Russia's foreign ministry said the international outcry suggested Navalny was working with foreign governments.

"This means that they are politically involved in the story," Maria Zakharova said, adding that Washington, with its response, was exposing "agents".

"They show such political zeal because it touches those whom they supervised, those whom they supported politically and in other ways," Ms Zakharova said.

Russian officials have repeatedly accused the opposition of working for and receiving funding from foreign interests.