A non-exploded Russian aviation bomb FAB-500 is seen next to a private house in Chernihiv, Ukraine. Reuters

Russian President Vladimir Putin on Saturday warned the West of a wider war if a no-fly zone is set up, as his forces resumed an offensive against a key Ukrainian city where a planned evacuation of residents failed to take place over security fears.

With the Kremlin’s rhetoric growing fiercer and a reprieve from fighting dissolving, Russian troops continued to shell encircled cities and the number of Ukrainians forced from their country grew to 1.5 million. By nighttime Russian forces had intensified their shelling of Mariupol, while dropping powerful bombs on residential areas of Chernihiv, a city north of Kyiv, Ukrainian officials said.

The head of the Chernihiv region said Russia dropped powerful bombs on residential areas of the city, which has a population of 290,000. Vyacheslav Chaus posted a photo online of what he said was an undetonated FAB-500, a 1,100-pound (500-kilogramme) bomb.

Volodymyr Zelensky criticising NATO for ruling out a no-fly zone for fear of sparking nuclear conflict.

"Usually this weapon is used against military-industrial facilities and fortified structures,” Chaus said.

The West has broadly backed Ukraine, offering aid and weapons and slapping Russia with vast sanctions. But the fight itself has been left to Ukrainians, who have expressed a mixture of courageous resolve and despondency.

"Ukraine is bleeding,” Foreign Minister Dmytro Kuleba said in a video released Saturday, "but Ukraine has not fallen.”

Putin also warned that Ukrainian statehood is in jeopardy and likened the West's sanctions on Russia to "declaring war,” while a promised cease-fire in the besieged port city of Mariupol collapsed amid scenes of terror.

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With his Ukrainian counterpart Volodymyr Zelensky criticising NATO for ruling out a no-fly zone for fear of sparking nuclear conflict, Putin spoke of "colossal and catastrophic consequences not only for Europe but also the whole world", if such a zone was set up.

"Any movement in this direction will be considered by us as participation in an armed conflict by that country," Putin said.

Ukrainian servicemen along with civilians carry a woman in Irpin, Ukraine, on Saturday. AP

Bereft mothers mourned slain children, wounded soldiers were fitted with tourniquets and doctors worked by the light of their cellphones as bleakness and desperation pervaded. Crowds of men lined up in the capital to join the Ukrainian military.

For Zelensky, on day 10 of the invasion, under an escalating bombardment that has flattened more and more infrastructure and sent nearly 1.4 million civilians fleeing for their lives, the Western military alliance's "no" to a no-fly zone had essentially given "the green light for further bombing of Ukrainian cities and villages".

A young child looks out of the window of a bus near the border crossing in Korczowa, Poland. AP

After Russia's defence ministry declared the ceasefire -- to open a humanitarian corridor out of the war's fiercest battles -- officials said the city's 450,000-strong population could begin to leave by bus and private cars.

However city officials then called a delay in the evacuation, saying: "The Russian side does not adhere to the ceasefire and has continued shelling both Mariupol itself and its environs, and for security reasons, the evacuation of the civilian population has been postponed."

Russia later announced the assault was back on.

Ukrainian servicemen help a woman carrying a small dog cross the Irpin river in Irpin, Ukraine. AP

"Due to the unwillingness of the Ukrainian side to influence nationalists or extend the ceasefire, offensive actions have been resumed," at 1500 GMT a Russian defence ministry spokesman said.

The siege came as more Russian forces inched closer to the capital.

The government has ordered men between the ages of 18 and 60 to stay and be available to fight. Some, like Volodymyr Onysko, have volunteered.

"We know why we are here. We know why we defend our country,” Onysko told Britain’s Sky News. "We know what we are doing, and that’s why we will win.”

Putin continued to pin the blame for the war squarely on the Ukrainian leadership and slammed their resistance to the invasion.

"If they continue to do what they are doing, they are calling into question the future of Ukrainian statehood,” he said. "And if this happens, it will be entirely on their conscience.”