Chechen leader Ramzan Kadyrov has been taken to hospital with suspected coronavirus, Russian media report.
He was flown to Moscow on Wednesday for treatment after developing flu-like symptoms, said Interfax news agency quoting a source.
"He's in hospital in Moscow," another source told the Ria Novosti agency.
The authoritarian leader - who previously fought against Moscow as a Chechen nationalist - is now a key ally of Russian President Vladimir Putin.
He is considered one of the most feared men in Russia, BBC Moscow correspondent Steve Rosenberg reports.
Mr Kadyrov has come under scrutiny for imposing his own interpretation of Islamic law in the region, as well as for his poor human rights record.
Unconfirmed reports suggested a plane belonging to the 43-year-old leader was tracked flying from near the Chechen capital, Grozny, to Vnukovo airport in Moscow on Thursday afternoon.
His condition is unknown, although initial reports suggested his doctor had recommended he travel to the Russian capital.
'In personal control'
A close ally of the Chechen leader, Grozny TV chief Akhmed Dudayev, was quoted by Tass news agency as saying Ramzan Kadyrov "is personally in control of the situation, he is taking all necessary measures. The work of his headquarters is under his personal control".
If the virus is confirmed, Mr Kadyrov would be the latest high-profile Russian official to contract the illness.
Prime Minister Mikhail Mishustin only returned to work in the last few days after being treated in hospital for Covid-19.
President Vladimir Putin's press secretary Dmitry Peskov also tested positive, as did culture minister Olga Lyubimova and construction minister Vladimir Yakushev - as well as Mr Yakushev's deputy, Dmitry Volkov.
Who is Kadyrov?
The Chechen leader controls the southern republic as if it were a personal fiefdom. His father, Akhmad Kadyrov, won a disputed presidential vote and was killed in a bomb attack not long afterwards in 2004. He is now the subject of a personality cult.
Under Ramzan Kadyrov's rule there have been repeated allegations of abuse at the hands of his security forces, with claims of extrajudicial killings and illegal detentions. In 2017, the BBC spoke to gay men who gave details of human rights abuses they had suffered.
Critics have linked him to assassinations, all denied. Five members of his security forces were jailed for the murder of Russian opposition politician Boris Nemtsov in 2015.
The Chechen leader has been strongly critical of medics in the republic who have complained of a lack of protective equipment.
"Provocateurs should be sacked. We have enough of everything - equipment, suits, masks, oxygen, medication," he told a meeting last Friday.
He has also called for people who filmed police detaining anyone violating lockdown restrictions to work as cleaners in hospitals or police stations.
The southern republic has tightened its lockdown measures this weekend, banning all travel for the Muslim festival of Eid al-Fitr.
How big is Russia's outbreak?
On Thursday Russia's reported death toll rose by 127 in the previous 24 hours to 3,099, with confirmed cases jumping to 317,554.
It is the second highest number of recorded infections worldwide. Critics believe the death toll is far higher than officially stated.
According to Chechen data, 1,026 people have been diagnosed in the republic and 11 have died. In neighbouring Dagestan dozens of medics are reported to have died and doctors speak of hospitals full of patients, but the number of confirmed cases is low.
The government in Moscow says the outbreak nationally is stabilising. The daily rise in new cases has dropped in recent days. On Wednesday, the number of people discharged from hospital after recovery for the first time exceeded the number of new infections over the previous 24 hours.