BERLIN -- Alexei Navalny's wife and a top aide visited him Sunday in a Berlin hospital where the comatose Russian dissident is being treated by German doctors after a suspected poisoning.

Navalny was flown to Germany on Saturday from Siberia after doctors determined he was stable enough to be brought to the capital's Charite hospital for treatment.

After his arrival, hospital spokeswoman Manuela Zingl said the 44-year-old would be undergoing extensive diagnostic tests and doctors wouldn't comment on his illness or treatment until they were able to evaluate the results.

On Sunday, Navalny's wife, Yulia Navalnaya, and aide Leonid Volkov visited the Russian opposition leader in the hospital but did not speak to reporters.

Navalny, a politician and corruption investigator who is one of Russian President Vladimir Putin's fiercest critics, fell ill on a flight back to Moscow from Siberia on Thursday and was taken to the hospital in the city of Omsk after the plane made an emergency landing. His supporters believe tea he drank was laced with poison -- and that the Kremlin is behind both his illness and a delay in transferring him to Germany.

While his supporters and family members insist Navalny was poisoned, doctors in Omsk denied that, saying a metabolic disorder was the most likely diagnosis and a drop in blood sugar may have caused Navalny to lose consciousness.

Russian health authorities on Saturday said tests so far haven't shown any poisons in his system.

When German specialists arrived Friday morning aboard a plane equipped with advanced medical equipment at his family's behest, Navalny's physicians in Omsk initially said he was too unstable to move. The dissident's supporters suggested that was just a ploy to delay his departure until any poison was out of his system.

The Kremlin denied resistance to the transfer was political, with spokesman Dmitry Peskov saying it was purely a medical decision. However, the reversal to allow him to leave came as international pressure on Russia's leadership mounted.

Navalny's own doctor, Yaroslav Ashikhmin, said the politician has always been in good health, regularly went for medical checkups and didn't have any underlying illnesses that could have triggered his condition.

Like many other opposition politicians in Russia, Navalny has been frequently detained by law enforcement and harassed by pro-Kremlin groups. In 2017, he was attacked by several men who threw antiseptic in his face, damaging an eye.

Last year, Navalny was rushed to a hospital from jail where he was serving a sentence on charges of violating protest regulations. His team also suspected poisoning then. Doctors said he had a severe allergic reaction and sent him back to detention the following day.

If he was poisoned, it would not be the first time a prominent, outspoken Russian was targeted in such a way -- or the first time the Kremlin was accused of being behind it.