The leader of the east Ukraine breakaway region of Donetsk on Saturday said he had introduced general mobilisation, 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.
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.
Kyiv has repeatedly denied any plans to regain control of separatist-held areas using force, including the Crimean peninsula, which was annexed by Moscow in 2014.
More than 14,000 people have been killed in fighting between Ukraine's army and Moscow-supported separatists since fighting broke out in 2014.