The UN Security Council will meet informally next week to discuss the plight of detained Russian opposition leader Alexei Navalny, at the risk of causing tension with Moscow, diplomats said Friday.

The meeting is scheduled to take place Wednesday after regular monthly discussions on the war in Syria. The session will be officially framed as a discussion on Navalny's poisoning last year, one diplomat said.

But the diplomat said some council members will probably bring up the current situation of Navalny, who was arrested January 17 after returning to Russia from Germany, where he had been recovering from a near fatal poisoning with a nerve toxin. 

Street protesters in Russia and many countries including the United States and the European Union have demanded his release from detention.

These countries probably want to stress "what the regime does to people who oppose it," this diplomat said on condition of not being named.

Russia cannot block the meeting because it is an informal one, behind closed doors, in which any council member can bring up any subject it wishes.

But Moscow is still expected to complain about the holding of the meeting and insist that the Navalny affair is not the UN's business because it does not threaten world peace or security.

"It's a matter of internal politics. That's why we do not tolerate, we will not tolerate any foreign interference into the situation," deputy Russian ambassador Dmitry Polyanskiy told a news conference this week.

Navalny allies under house arrest ahead of new Russia protests

Key allies of jailed Kremlin critic Alexei Navalny were placed under house arrest ahead of new rallies, as Russian authorities warned Friday that protesters could face charges of taking part in "mass unrest".

Navalny's brother Oleg and the coordinator of Navalny's Moscow office, Oleg Stepanov, were placed under house arrest until March 23 after the opposition politician urged Russians to stage fresh nationwide rallies against President Vladimir Putin's 20-year rule on Sunday. 

Prominent activist Lyubov Sobol's lawyer said the freedom of the 33-year-old -- who has a small child -- could also be restricted.

The pressure on Navalny's family and associates grew after tens of thousands of Russians rallied last weekend in support of Putin's most vocal domestic critic, who survived a near-fatal poisoning with a nerve agent. 

More than 4,000 protesters were detained across the country and authorities launched a number of criminal probes in the aftermath of the demonstrations.

Several Navalny associates, including Sobol, were detained following police raids on their apartments and offices this week. They are accused of violating restrictions related to the coronavirus pandemic by calling for people to join protests.

Moscow lifted a raft of coronavirus restrictions earlier this week, allowing bars and restaurants to work all night and offices to be fully staffed, citing a steep decline in infections.

However, city officials refrained from lifting a ban on mass gatherings. 

The Investigative Committee, which probes major crimes, also announced Friday that Leonid Volkov, the head of Navalny's regional network based in the Lithuanian capital Vilnius, was wanted on charges of calling minors to join unauthorised rallies.