Moscow, Russia - Amid massive tensions between Russia and the United States, US Ambassador John J. Sullivan has left Moscow to retire.

The diplomat, appointed in December 2019 by then US president Donald Trump, has completed his assignment after nearly three years, the US embassy in Moscow announced on Sunday.

According to the statement, the 62-year-old is ending his career after four decades in public service under five US presidents.

Sullivan, who had repeatedly sharply criticized Russian policy, had also served as US deputy secretary of state.

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Until the new ambassador arrives, Sullivan's deputy, Elizabeth Rood, will conduct business at the embassy, it said.

On Saturday, Sullivan had paid his last respects to the late Nobel Peace laureate Mikhail Gorbachev in Moscow with thousands of mourners. The diplomat repeatedly addressed authoritarian tendencies, judicial arbitrariness, and the violation of human rights in Russia.

Last year, Sullivan had already returned to his home country for three months following an escalation of tensions between Russia and the US.

He only resumed his official duties in the Russian capital after a summit meeting between Russian leader Vladimir Putin and US President Joe Biden in Geneva in June last year.

In the course of sanctions and counter-sanctions, the US and Russia have had to massively reduce staffing at their embassies.