Russia on Friday warned Britain against violating fundamental diplomatic principles after London suggested it could arrest WikiLeaks founder Julian Assange inside Ecuador’s embassy.

“What is happening gives grounds to contemplate the observance of the spirit and the letter of the Vienna Convention on Diplomatic Relations, and in particular the Article 22 spelling out the inviolability of diplomatic premises,” the Russian foreign ministry said.

Ecuador on Thursday granted asylum to Assange – whose website enraged the United States by publishing a vast cache of confidential government files – but Britain has vowed not to grant him safe passage out of the country.

British Foreign Secretary William Hague has said his government was obliged under its own law to extradite the Australian national to Sweden, where he is wanted for questioning over alleged sex crimes.

Britain has angered Ecuador by suggesting it could invoke a domestic law allowing it to breach the usual rules and go in to arrest Assange, who has been holed up in Ecuador’s London mission since June.

This would challenge a fundamental principle of the diplomatic system, and the threat has left Britain in unchartered legal waters.

At the same time, Moscow warned Britain against interpreting the law selectively, stressing that London has given refuge to “dozens of people suspected of committing grave crimes” who are wanted in other countries.

“What to do with a right to refuge for Julian Assange when London turns the observance of this right for this category of people into an absolute principle?” the Russian foreign ministry asked, referring to a number of high-profile figures granted asylum in Britain.

Russia has for years sought the extradition of top Kremlin critic Boris Berezovsky as well as several other figures.