Moscow-backed separatists in eastern Ukraine are urging civilians to flee across the border to Russia to protect themselves from an attack by Ukrainian government forces.
The call for an exodus to Russia came as the rebels and authorities in Kiev traded blame for an escalation in violence along a ceasefire line, while Western powers repeated their fears that an invasion by Russian forces could be at hand.
"Women, children and elderly people" should first be brought to safety, Denis Pushilin, the head of self-proclaimed Donetsk People's Republic, said in a video message.
Pushilin said he was making an "urgent request" for residents to make a "mass, centralized departure," citing the threat of impending military action by Ukrainian troops.
In consultation with the Russian authorities, shelters had been provided in the neighbouring Rostov region, he said.
Sirens could be heard wailing in videos from Donetsk recorded in the afternoon. Evacuation plans for the town of Horlivka, for example, were also published. According to these plans, the first buses were to leave at around 1730 GMT. People were told to take only the most necessary things with them, such as documents, money, a change of clothes and medicine.
Pushilin said combat troops staying behind were ready to defend "state territory" against an attack by Ukraine. "We will be victorious," he said at the end of the video.
The separatists in nearby Luhansk, the other rebel stronghold in Ukraine, also ordered similar measures.
Kiev has repeatedly denied having any military plans to move on the separatists.
Other countries, meanwhile, were reiterating their positions that it is Russia imperilling the region's security.
The US envoy to the Organization for Security and Co-operation in Europe (OSCE) said that Ukraine is threatened by 169,000 to 190,000 troops and security forces under Russian control.
"Colleagues, this is the most significant military mobilization in Europe since the Second World War," Ambassador Michael Carpenter told the OSCE in Vienna.
The US estimate includes soldiers in the Russian border region, Belarus and the Ukrainian peninsula of Crimea annexed by Russia, the envoy said.
Adding to the alarm were fresh claims of shelling over the past 24 hours in Ukraine's Donbas region, where the rebel-held areas of Donetsk and Luhansk are located.
Separatists there said government forces had intensified shelling overnight and that two transformer stations had been damaged.
The Ukrainian army alleged Friday morning there had been almost two dozen violations by the rebels of the ceasefire agreed to in 2015 under Franco-German mediation.
The claims could be not be independently verified.
Moscow is "very concerned" about increased shelling in eastern Ukraine, Russian Foreign Minister Sergei Lavrov said, alleging that Ukrainian troops were using weapons banned under the 2015 peace plan.
On Thursday a kindergarten in government-controlled territory was directly hit by separatist shelling, according to Kiev.
Western powers have warned for weeks that the region could be on the precipice of war, and the incident at the kindergarten ignited fears that the situation could escalate quickly and dramatically.
The strike on the school was interpreted by some leaders, including British Prime Minister Boris Johnson, as a strategic provocation in order to prompt a counter-attack by Kiev that would offer Russia a pretext to launch an invasion.
German Chancellor Olaf Scholz and French President Emmanuel Macron were among the leaders on Friday calling for dialogue and the laying down of weapons.
Russia has repeatedly denied having any intension of attacking its neighbour. The government condemns US, NATO and European assessments of the situation as distorted and an attempt to whip up anti-Russia "hysteria."
Meanwhile, the Kremlin said President Vladimir Putin will oversee military drills on Saturday involving the launch of ballistic and cruise missiles.
Kremlin spokesperson Dmitry Peskov said Russia was being transparent about the exercises, had given notice to all proper channels, and that they "should not cause anyone concern."
Elsewhere, Russian Defence Minister Sergei Shoigu and his US counterpart Lloyd Austin spoke by phone on Friday.
Austin said called for "deescalation, the return of Russian forces surrounding Ukraine to their home bases, and a diplomatic resolution," a Pentagon spokesperson said.
The US defence secretary was on Friday in Poland, where he warned that a Russian invasion of Ukraine could trigger an influx of "tens of thousands" of people trying to escape to the safety in the European Union.
Polish Defence Minister Mariusz Blaszczak said his country, which borders Ukraine, was ready to help those forced to leave in the event of a Russian attack. Last week, Poland said emergency shelters were being prepared.
Austin also said the US plans to provide Poland with 250 M1A2 Abrams tanks but the time frame for the delivery was still being clarified.