Julian Assange has launched legal action against the government of Ecuador, arguing that new house rules for his stay in the country’s London embassy violate his “fundamental rights and freedoms”, his lawyers said on Friday.

Assange’s lawyer, Baltasar Garzón, said the WikiLeaks founder was suing Ecuador’s foreign minister, José Valencia, for “isolating and muzzling” him with the new rules, which order Assange to avoid making online political comments and to take better care of his cat.

“He has been held in inhuman conditions for more than six years,” Garzón said, describing the conditions regarding the cat as “denigrating”.

He added Assange had not had his internet restored even though the new rules allow him to use the embassy wifi for his personal computer and phone.

The move follows a deterioration in relations between the Ecuadorian government and the WikiLeaks founder, who was granted refuge at Ecuador’s London embassy in 2012 while on bail in the UK over sexual assault allegations against him in Sweden.

Valencia said the country would respond to the lawsuit and that it had the “absolute prerogative and right” to protect its offices, workers and Assange himself inside its embassy.

“Protocol has to be respected, whether they like it or not. It’s a norm to regulate Mr Assange’s stay in our embassy,” he told journalists, adding that the rules met with international standards and Ecuadorean laws.

Garzón said the new rules had been imposed “unilaterally” and WikiLeaks had not been properly consulted about its contents.

Newly-released Ecuadorian government documents this week laid bare an unorthodox attempt to extricate the WikiLeaks founder from his embassy hideaway in London by naming him as a political counsellor to the country’s embassy in Moscow.

Under the new rules, Assange must obtain approval for all visitors from diplomatic staff three days in advance. He is banned from activities that could be “considered as political or interfering with the internal affairs of other states”, according to a memo seen by the Guardian.

In a statement posted on Friday, WikiLeaks said Ecuador was now a “strategic ally” of the United States and there was a heightened risk of extradition for Assange.

Earlier this week, US congressmen wrote an open letter to Ecuador’s president, Lenín Moreno, which stated that to advance “crucial matters ... from economic co-operation to counter-narcotics assistance to the possible return of a USAID mission to Ecuador, we must first resolve a significant challenge created by your predecessor, Rafael Correa – the status of Julian Assange”.

The Ecuadorian government partially lifted restrictions on Assange’s internet access last weekend, but stipulated he would only be allowed to use the embassy wifi for his personal computer and phone.

The WikiLeaks Twitter account stated on Thursday that, “after US pressure”, moves had accelerated to strip Assange of Ecuadorian citizenship. “His citizenship status is a barrier to rendering him to another state as article 79 of Ecuador’s constitution forbids extradition of citizens,” it added.

Assange was made an Ecuadorian citizen last December in an attempt by the nation’s foreign ministry to resolve the political impasse over his continued presence in the UK. The 47-year-old was naturalised after living for five-and-a-half years in the Latin American country’s cramped embassy in Knightsbridge, central London.

Earlier this year, the UK Foreign Office revealed Ecuador had asked for Assange, who was born in Australia, to be accredited as a diplomat. The request was dismissed.

The Ecuadorian initiative was intended to confer legal immunity on the WikiLeaks founder, allowing him to slip out of the embassy and Britain without being arrested for breaching his former bail conditions.