Russians started voting on Friday in a three-day presidential election that is set to hand veteran leader Vladimir Putin another six-year term as the raging conflict in Ukraine spreads further into Russian territory.

In power as president or prime minister since the final day of 1999, the former KGB agent is casting the election as a show of Russians' loyalty and support for his military assault on Ukraine, now in its third year.

Polling stations in a country spread over 11 time zones opened at 8:00 am on Friday (2000 GMT  Thursday) on the Far Eastern Kamchatka peninsula and will close Sunday at 8:00 pm (1800 GMT) in Russia's Kaliningrad exclave, wedged between EU members Poland and Lithuania.

Putin on Thursday had urged Russians to back him in the face of a "difficult period" for the country, in a pre-election message broadcast on state TV.

"We have already shown that we can be together, defending the freedom, sovereignty and security of Russia ... Today it is critically important not to stray from this path," he said.

The Kremlin leader's confidence is riding high with his troops recently having secured their first territorial gains in Ukraine in nearly a year.

At home, his most strident and charismatic critic of the last decade, Alexei Navalny, died in an Arctic prison colony last month. He had been serving 19 years on "extremism" charges widely seen as retribution for his campaigning against the Kremlin.

- 'Farce' -

With all of Putin's major opponents dead, in prison or in exile, the outcome of the vote is not in any doubt.

A state-run pollster predicted earlier this week that Putin would secure more than 80 percent.

Victory will allow him to stay in power until 2030, longer than any Russian leader since Catherine the Great in the eighteenth century.

Rights groups say the poll will be neither free nor fair.

Election authorities barred the few genuine opposition candidates who tried to run against him.

But those who oppose Putin still hope to spoil the procession. Navalny's widow, Yulia Navalnaya, is among those calling for voters to show up outside polling stations at midday on Sunday, the final day of voting, as a form of protest.

Moscow prosecutors warned "the organisation of and participation in these mass events are punishable."

Voting was also being staged in occupied parts of eastern Ukraine that Russia claims to have annexed.

Armed soldiers in full combat gear accompanied election officials in the eastern Donetsk region as they set up mobile voting stations on small tables in the street and on the hoods of Soviet-era cars.

Kyiv has branded the vote as a "farce" and said staging the election in eastern Ukraine and Crimea, which Russia annexed in 2014, was "illegal."

US State Department spokesman Matthew Miller also condemned the voting in Russian-held areas on Thursday.

"The United States does not and will never recognise the legitimacy or outcome of these sham elections held in sovereign Ukraine," he said.

Moscow has promoted the vote using a red, white and blue 'V' logo, a symbol associated with its military offensive on Ukraine.

- Ukrainian attacks -

Kyiv has launched some of its largest air attacks on Russia this week ahead of the election, with some reaching hundreds of kilometres into Russian territory.

Shortly after polling stations opened on Friday morning, Russia's defence ministry said it had "destroyed seven rocket-launched shells over the Belgorod region," on the border with Ukraine.

The state-run RIA Novosti news agency said voters were forced to leave a polling station in the region to head to a bomb shelter as authorities issued an air alert and ordered people to take cover.

Pro-Ukrainian militias -- made up of Russians who oppose the Kremlin and have taken up arms for Kyiv -- also launched a wave of guerilla-style attacks on border areas this week, claiming to have seized territory inside Russia.

Moscow has rejected those claims and said it hit the combatants with a wave of artillery and rocket fire, thwarting their attempts.