Ukraine is restricting the entry of Russian men aged 16 to 60 on the fear that Russia could form "units of private armies," Ukrainian President Petro Poroshenko said late Thursday.

The measure is to "prevent Russians from implementing in Ukraine the operations that they planned in 2014," Poroshenko said on Twitter, referring to the outbreak of a pro-Russian separatist rebellion in eastern Ukraine four years ago.

Poroshenko said that such militia units are "in reality representatives of the armed forces of the Russian Federation."

More than 10,000 people, including 2,700 civilians, have been killed in the conflict between Russia-loyal separatist groups and the Ukrainian military, according to estimates by the United Nations.

Russian President Vladimir Putin has represented the separatists in international negotiations, but his government denies providing them active servicemen or weaponry.

Tensions between Ukraine and neighbouring Russia escalated last weekend when the Russian coastguard opened fire and captured several Ukrainian naval vessels near the Russian-annexed Crimea region, which Ukraine maintains is its territory.

Ukrainian Prime Minister Volodymyr Groysman called for harsher EU sanctions against Russia in response to the naval clash, according to an interview published on Friday.

"Russia is an aggressor and an occupier," Groysman told German newspaper Die Welt.

Groysman also criticized the planned expansion of a natural gas pipeline linking Russia and Germany - Nord Stream 2 - asserting that it drives the European Union towards dependence on Russia.

"It is not just harmful for Ukraine, but for the whole continent," Groysman told the newspaper.

Ukraine is set to lose billions of dollars in transit fees as the gas corridor does not pass through its territory.

German Economy Minister Peter Altmaier has warned that the pipeline issue should not be mixed up with the Crimea tensions.

"Those are two different areas," Altmaier, a member of Chancellor Angela Merkel's Christian Democratic Union, told German public broadcaster ARD.