As tensions spiralled and fears of a Russian invasion grew, the Ukrainian military said a soldier was killed in shelling by pro-Moscow separatists in eastern Ukraine.
Russian-backed separatist leaders in eastern Ukraine have declared a full military mobilisation, a day after ordering women and children to evacuate to southern Russia.
Denis Pushilin, head of the self-proclaimed Donetsk People's Republic, said in a video statement on Saturday that he had signed a decree on mobilisation.
"I urge my fellow citizens who are in the reserves to come to military conscription offices," Pushilin said, calling on men "able to hold a weapon in their hands" to come to military commissariats.
Pushilin claimed his region's forces prevented attacks 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."
Another separatist leader, Leonid Pasechnik, signed a similar decree for the Luhansk People's Republic shortly afterwards.
Incidents of shelling across the line dividing government forces and separatists increased sharply this week, in what the Ukrainian government called a provocation.
The Ukrainian military said a soldier was killed in shelling by pro-Russian separatists on Saturday.
"As a result of a shelling attack, one Ukrainian soldier received a fatal shrapnel wound," the joint military command for east Ukraine reported.
The military said it had recorded 12 ceasefire violations by the rebels — who seized a swath of territory in eastern Ukraine in 2014 — on Saturday morning after 66 cases over the previous 24 hours.
Monitors from the Organization for Security and Co-operation in Europe said they had seen a significant rise in the number of attacks along the front line, particularly in Donetsk and Lugansk.
The latest upticks in violence involving artillery and mortar attacks are being watched around the world, with the United States and its NATO allies alleging that the Kremlin is looking for an excuse to invade Ukraine.
On Friday, separatist authorities in Donetsk and Luhansk announced a mass evacuation of women, children and the elderly to neighbouring Russia, exacerbating an already febrile situation.
Officials told local media that 25,000 people had left Lugansk and more than 6,000 had left Donetsk for Russia.
The US says that with an estimated 149,000 Russian troops on Ukraine's borders — as many as 190,000, when including the Russian-backed separatist forces — an attack is inevitable.
The Kremlin continues to say it has no plans to attack.