The high-stakes negotiations come amid fears of a Russian invasion of its pro-Western neighbour Ukraine.

"As a result of an explosion of an unknown explosive device, two servicemen received injuries incompatible with life," the army said.

In a statement on Facebook, the army accused separatists of firing at its positions with grenade launchers, machine guns and small arms.

Monday's talks in Geneva launch a week of diplomacy between Russia and the West after Moscow amassed tens of thousands of troops at the Ukrainian border, drawing the United States into a Cold War-style stand-off.

Both sides have set firm lines, with Washington warning of severe diplomatic and economic consequences from an invasion of Ukraine, and Russia demanding wide-ranging concessions from the West.

US Secretary of State Antony Blinken stressed Sunday that Russia had to choose between dialogue and confrontation.

Western officials have warned Moscow it could also see NATO boost its presence near Russia's borders.

Moscow insists it was deceived after the Cold War and understood that NATO would not expand eastward.

Russia has put intense pressure on Ukraine since 2014 after a revolution overthrew a government that had sided with the Kremlin against moving closer to Europe.

Russia seized the Crimean peninsula and backs an insurgency in eastern Ukraine in which more than 13,000 people have died.