Russian ultra-nationalist Igor Strelkov has said President Vladimir Putin could be toppled "like the last tsar" if he does not fully support pro-Russia separatists in eastern Ukraine.

According to the Sunday Times, the former commander of the pro-Russia rebels in the disputed region speculated about Putin's removal from power when he addressed supporters last week.

Although Putin is seen as a hardliner in the West, Strelkov believes the Russian premier is too liberal, and has failed to act decisively enough in the region due to his decision not to annex Donetsk and Luhansk.

"Ukraine was, is and remains a part of Russia," he said according to the newspaper. He warned that Putin could "end up like [former Serbian leader Slobodan] Milošević who was toppled by both patriots and liberals over his policy of being neither with one group nor the other."

This sentiment was also expressed in a controversial interview with German paper Der Speigel published 18 March, in which Strelkov spoke of his longing for the re-emergence of a Russian empire led by a totalitarian leader.

Speaking with a portrait of Tsar Nicholas II – who was murdered with his family by the Bolsheviks – propped on his desk, Strelkov speculated that the "Russian world" should include Belarus, Georgia, Armenia, and possibly Central Asia.

"The real separatists," he told the newspaper, "are the ones in Kiev, because they want to split Ukraine off from Moscow."

Strelkov, whose real name is Igor Vsevolodovich Girkin, first came to prominence in April 2014, when he led a group of pro-Moscow separatists into the east Ukrainian city of Sloviansk, sparking the large-scale uprising against the government in Kiev.

It later emerged that he had served with the FSB, the Russian intelligence agency that replaced the KGB, for 18 years. When he retired in 2013, he was part of the Directorate for Combating International Terrorism. Strelkov is also a veteran of both the Soviet and Russian armies, and was charged with terrorism offences by the Ukrainian authorities during the current conflict.

In November last year, Strelkov said he took responsibility for triggering war in eastern Ukraine.

He has also established the Novorossiya movement, which supports rebels in the Crimea. The group's name, meaning "New Russia", is a historical term for southern and eastern Ukraine used by the Russian Empire.

The European Union added the group to its sanctions list in February this year.