Saturday morning, the Ukrainian army said that a soldier was killed in shelling by pro-Russian separatists in east Ukraine.

This came at a time when the leader of the east Ukraine breakaway region of Donetsk on Saturday said he had introduced general mobilization, with fears mounting of war in the ex-Soviet country.

The announcement came after observers from the Organization for Security and Cooperation in Europe reported a significant rise in attacks on the frontline in eastern Ukraine, controlled by pro-Russian rebels.

"I urge my fellow citizens who are in the reserves to come to military conscription offices. Today I signed a decree on general mobilization," Denis Pushilin, the leader of the so-called Donetsk People's Republic, said in a video statement.

Western leaders have for weeks raised the alarm over a build up of Moscow's army around Ukraine with Washington warning that an imminent attack could take place.

Pushilin claimed his region's forces had prevented attacks he said were planned by Ukraine, and that the Ukrainian army had continued attacks.

"Together, we will achieve for all of us the victory we desire and need. We will protect Donbas and all Russian people," Pushilin added.

Kyiv has repeatedly denied any plans to regain control of separatist-held areas using force, including the Crimean peninsula.

More than 14,000 people have been killed in fighting between Ukraine and Moscow since fighting broke out in 2014.