The team of U.S. President-elect Joe Biden on Sunday called for the release of Russian dissident Alexei Navalny after he was detained upon arrival at a Moscow airport, as the Trump administration remained quiet on the subject.

“Mr Navalny should be immediately released, and the perpetrators of the outrageous attack on his life must be held accountable,” said Jake Sullivan, who will serve as Biden’s national security adviser if confirmed by the Senate.

“The Kremlin’s attacks on Navalny are not just a violation of human rights, but an affront to the Russian people who want their voices heard.”

The Kremlin critic was travelling home after spending five months in Germany, where he received emergency medical treatment following his poisoning with the nerve agent Novichok.

The 44-year-old was detained while going through passport control and is to be remanded in custody until a court decision, with a trial scheduled for Jan. 29.

Russian authorities accuse Navalny of breaking the terms of a suspended sentence and probation.

Corrections authorities are, therefore, seeking to transfer those into a jail sentence.

The French Ministry for Europe and Foreign Affairs said it was following the situation “with the utmost vigilance” as it called for Navalny’s release.

European Council President Charles Michel and European Union’s (EU) foreign affairs chief Josep Borrell earlier also condemned the dissident’s arrest and called for his release.

“The detainment of Alexey #Navalny upon arrival in Moscow is unacceptable,” Michel tweeted.

“I call on Russian authorities to immediately release him.

“Russian authorities must respect Alexei @navalny’s rights and release him immediately,” wrote Borrell.

“Politicisation of the judiciary is unacceptable,” he added.

The EU officials’ words were echoed by Baltic countries Estonia, Latvia, and Lithuania in a joint statement late on Sunday.

“Detaining Alexei Navalny by the Russian authorities is completely unacceptable.

“We demand his immediate release,” the statement by the three countries’ foreign ministers said, as they also called on the EU to “act swiftly and if he is not released, we need to consider imposition of restrictive measures in response to this blatant act.”

Amnesty International called the arrest “further evidence that Russian authorities are seeking to silence him.”

“His detention only highlights the need to investigate his allegations that he was poisoned by state agents acting on orders from the highest levels,” said Amnesty International’s Moscow office director Natalia Zviagina.

“The authorities handled Aleksei Navalny’s arrival as a fully blown security operation with hundreds of police officers summoned to the Vnukovo Airport, arresting and pushing out Navalny’s supporters,” Zviagina added.