Detained upon arrival at an airport in Moscow on Sunday, the Russian opposition leader Alexei Navalny was the main reason for the statement of the National Security Advisor of President-elect Joe Biden, Jake Sullivan, as he calls for the held Putin critic's release.

Navalny flew to Russia from Berlin, earlier on Sunday, after he had spent nearly half a year in Germany, recovering since he was poisoned last summer.

The Putin critic's release call was mentioned right away after Navalny was arrested at passport control on Sunday.

According to CNBC, the Russian authorities last week, they have issued a warrant for the arrest of Navalny, which claims that the Putin critic had violated the terms of a suspended three-and-a-half-year sentence he received back in 2014 for charges of embezzlement.

Sullivan tweeted the Putin critic's released immediately, while also mentioning that the perpetrators of the outrageous attack on his life must be held accountable.

Meanwhile, when asked for a comment, The White House and the Department of State did not respond immediately to the request.

Sullivan's call of the release of Navalny comes days before the Inauguration of President-elect Joe Biden, as he is about to swear into office.

The incoming administration of Biden is expected to increase pressure on Russia.

During the wake of the poisoning of Navalny last year, Biden vowed to work with their partners and allies to hold the Putin regime accountable for its crimes while also accusing President Donald Trump of not taking a tough enough stance.

In response to the poisoning of Navalny, a bipartisan group of United States senators had called on the Trump administration is imposing sanctions on Russia. However, Trump, who will be leaving the office on Wednesday, has not done so.

The EU or European Union and the United Kingdom, which are close allies of the United States, moved swiftly in imposing targeted sanctions on six Russians along with a state research center in October.

Read Also: Inauguration Security: Police Apprehend Individuals Violating DC Security Protocol

On his flight back to Moscow, the Putin critic shared to reporters that he felt great and that making the journey home has been the best moment of his life in the last five months.

According to Reuters, Navalny shared that he felt great as he will be returning his hometown, finally.

The Poisoning

The Putin critic was medically evacuated to Germany from a Russian hospital last year after becoming ill following reports that something was added to his tea.

During that time in Russia, the doctors who are treating Navalny denied that the critic of the Kremlin had been poisoned and blamed the said comatose state on Navalny's low blood sugar levels.

In September, the German government stated that the 44-year-old Russian dissident was poisoned by a chemical nerve agent, wherein they described the toxicology report as providing unequivocal evidence.

The nerve agent was in the family Novichok, which was developed by the Soviet Union.

Moreover, on the heels of the tests conducted, the White House also mentioned that it was deeply troubled by the matter and called the poisoning as completely reprehensible, The Guardian reported.

Related Article: Pablo Escobar's 'Cocaine Hippos' Uncontrollable Breeding Pose Ecological Threat