A current focus of the crisis is in eastern Ukraine where Russian-backed rebels seized a swathe of territory in 2014, the same year that Moscow annexed Ukraine's Crimea region after protests there toppled a pro-Russian leader. Kyiv says more than 14,000 people have since died in the conflict in the east.
Russian-backed separatist leaders in eastern Ukraine declared a full military mobilization on Saturday, a day after ordering women and children to evacuate to southern Russia because of what they said was the threat of conflict.
Denis Pushilin, head of the self-proclaimed Donetsk People's Republic, said in a video statement that he had signed a decree on mobilization and called on men "able to hold a weapon in their hands" to come to military commissariats.
Another separatist leader, Leonid Pasechnik, signed a similar decree for the Luhansk People's Republic shortly after.
Separatist authorities on Friday announced plans to evacuate around 700,000 people, citing fears of an imminent attack by Ukrainian forces - an accusation Kyiv flatly denied.
Less than 7,000 people had been evacuated from Donetsk as of Saturday morning, the local emergencies ministry said.
In one breakaway region, Denis Pushilin, head of the self-proclaimed Donetsk People's Republic, said he had signed a decree on mobilization and called on men "able to hold a weapon in their hands" to come to military commissariats.
Another separatist leader, Leonid Pasechnik, signed a similar decree for the Luhansk People's Republic shortly afterwards.
Separatist authorities on Friday announced plans to evacuate around 700,000 people. Russian news agencies said on Saturday 10,000 evacuees had arrived so far in Russia.
At a market in Donetsk, 38-year-old Oksana Feoktisova boarded an evacuation bus with her 9-year-old son and her mother. They were accompanied by Feoktisova's brother Yuri who stayed behind in Donetsk.
"They don't let men on, and I wouldn't go anyway frankly," Yuri said. "I'm a reservist in any case. I'm an artillery man by birth... I'm loyal to my state, to my people."
Shelling across the line dividing government forces and separatists increased sharply this week, in what the Ukrainian government called a provocation.
A jeep exploded outside a rebel government building in the city of Donetsk on Friday and Russian news agencies said two explosions hit Luhansk and part of a gas pipeline in the area caught fire.
The Ukrainian military also said two soldiers were killed in shelling by pro-Russian separatists on Saturday morning in east Ukraine.
Ukraine's military also said on Saturday it had recorded 12 ceasefire violations by pro-Russian separatists in eastern Ukraine in the morning after 66 cases in the previous 24 hours.
Separatist authorities also reported what they said was shelling by Ukrainian forces of several villages on Saturday.
Separatists opened fire on more than 20 settlements, using heavy artillery, which has been prohibited by Minsk agreements, the military said.
The Minsk accords aim to end an 8-year-long conflict between the Ukrainian army and separatists in the east of the country.
Both sides regularly trade blame for ceasefire violations.
The head of a Russia-dominated military alliance that is sometimes called Moscow's answer to NATO has said his organization could send peacekeepers to territory in eastern Ukraine controlled by Russia-backed rebels if needed.
The situation in two self-proclaimed pro-Russian republics in Ukraine's Donbass region is on a knife-edge after rebel leaders declared a full mobilization of their forces and asked civilians to evacuate to Russia following shelling which Ukraine and the separatist rebels blame on each other.
In an interview conducted before that evacuation began, Stanislav Zas, secretary-general of the Moscow-based Collective Security Treaty Organization (CSTO), said the body could despatch peacekeepers to Donbass if there was an international consensus for such a deployment.
"Hypothetically you can imagine it (such a deployment) if there were goodwill from Ukraine - it is after all their territory - if there was a UN Security Council mandate, and if it was needed and such a decision was supported by all our governments," Zas, a Belarusian lieutenant-general, told Reuters in what aides said was his first Western media interview.
Zas, whose organization last month deployed troops in the former Soviet Union for the first time in its 20-year history to help quell an attempted coup in Kazakhstan, said the only way to resolve the crisis in Ukraine was through negotiations.
"I don't believe that the situation now will return to how it used to be when tanks were housed at their bases and soldiers in their barracks. It won't happen. There needs to be an understanding about the necessity to sit down at the table and agree something. It's a new reality," said Zas.
The CSTO draws on the armed forces of its six members: Russia, Belarus, Armenia, Kazakhstan, Kyrgyzstan and Tajikistan. Russia, with overall active forces of 900,000, is by far the CSTO's most powerful member, and the organization's unified military headquarters is headed by a Russian.
Zas said the CSTO had a 17,000-strong force at constant readiness and a specialized peacekeeping force of nearly 4,000 troops.
A CSTO deployment to Donbass seems unlikely for now, given the organization's geopolitical allegiance and a view in the West that it is a tool of Russian influence.
But the situation in eastern Ukraine is changing fast as Moscow attempts to compel Kyiv to hold direct talks with the rebels and grant the two regions they control a measure of autonomy within Ukraine.
RUSSIAN DEMANDS
Backing its demands with what according to US estimates is up to 190,000 troops surrounding Ukraine on three sides, Russia also wants the idea of NATO membership for Ukraine taken off the table forever.
Kyiv has long spurned all those demands, viewing them as a ploy to give Russia influence over Ukraine's foreign and domestic policy via the rebels and as an attempt to thwart its turn to the West away from Moscow.
Zas rejected the idea that the CSTO was a vehicle to promote Russian influence. He predicted East-West tensions would remain dangerously high until a deal was done on the security guarantees Moscow is seeking from the West, which he described as being in the interests of the wider region and the CSTO.
Among other demands, Moscow wants NATO to halt its eastward expansion and promise not to deploy offensive weapons in Ukraine and other nations close to Russia.
Zas said the CSTO and its members would always carefully weigh any decision to use force.
"We have colossal potential in our hands. We all understand that we need to be very careful with this scalpel," he said.
If needed, the CSTO could swiftly deploy huge numbers of troops, he said.
"Believe me, we can send as many as needed," Zas said. "If we need 300,000, we'll send them. If we need 17,000 we'll send them. If we need more there'll be more. As many as are needed."
British Prime Minister Boris Johnson and German Chancellor Olaf Scholz met on Saturday and agreed on the need for more diplomacy to stop Russian leader Vladimir Putin from ordering an invasion of Ukraine, and strong responses if Russia does attack.
"The Prime Minister and Chancellor Scholz resolved to redouble efforts to reach a diplomatic resolution to prevent unnecessary bloodshed," Johnson's office said in a statement.
"Should President Putin make the catastrophic miscalculation to further breach Ukrainian sovereignty, allies must put in place a comprehensive response which strikes at the heart of Russia's strategic interests," the statement added.