A file photo dated September 29, 2019 shows Russian opposition leader Alexei Navalny during a rally in support of political prisoners on Prospekt Sakharova Street in Moscow, Russia. Alexei Navalny is passed out in hospital after allegedly being poisoned, according to his press secretary.

Sefa Karacan | Anadolu Agency via Getty Images

WASHINGTON – President-elect Joe Biden’s national security adviser Jake Sullivan called for the immediate release of Russian opposition leader Alexei Navalny, who was arrested at a Moscow airport on Sunday after his arrival.

The previous Sunday, Navalny flew from Berlin to Russia, where he had recovered for almost six months since being poisoned last summer. He was arrested at passport control.

Last week, Russian authorities issued an arrest warrant for Navalny alleging that he had violated the three and a half year suspended sentence he received in 2014 for embezzlement.

“Mr. Navalny should be released immediately and the perpetrators of the outrageous attack on his life must be brought to justice,” Sullivan wrote on Twitter.

The White House and State Department did not immediately respond to CNBC’s request for comment.

Sullivan’s call for Navalny to be released comes days before President-elect Joe Biden takes office. Biden’s new government is expected to increase pressure on Russia.

After the poisoning of Navalny last year, Biden vowed “to work with our allies and partners to hold the Putin regime accountable for its crimes,” and accused President Donald Trump of not being tough enough.

A non-partisan group of US senators had urged the Trump administration to impose sanctions on Russia in response to the poisoning of Navalny. Trump, who is leaving office on Wednesday, did not do so.

The United Kingdom and the European Union, close allies of the United States, swiftly imposed targeted sanctions on six Russians and a government research center in October.

On the return flight to Moscow, Navalny told reporters that he was feeling great and that the trip home was “the best moment in five months.”

“I feel great. I’m finally going back to my hometown,” he said, according to a Reuters report.

Last year, Navalny was medically evacuated to Germany from a Russian hospital after falling ill after reports that something had been added to his tea. Russian doctors treating Navalny denied that the Kremlin critic had been poisoned, blaming his comatose condition for low blood sugar levels.

In September, the German government announced that the 44-year-old Russian dissident had been poisoned by a chemical agent on nerves and described the toxicological report as “clear evidence”. The nerve agent was in the Novichok family, which was developed by the Soviet Union.

Following the test results, the White House said it was “deeply concerned” by the matter and called the poisoning “utterly reprehensible.”

“The United States is deeply concerned about the results released today,” White House National Security Council spokesman John Ullyot said in a written statement at the time. “The poisoning of Alexei Navalny is completely reprehensible. Russia has used the chemical nerve agent novichok in the past,” he said, referring to the poisoning of Sergei Skripal and his daughter in England in 2018.

The Kremlin has repeatedly denied a role in the poisoning of Navalny and Skripal.

Navalny’s arrest Sunday faces another strain on relations between European leaders and Russian President Vladimir Putin and comes while the Kremlin works to secure a gas pipeline project, Nord Stream 2, to Germany.