An evacuation of some of the 400,000 residents trapped in a key Ukrainian city by encircling Russian forces has been agreed under a temporary ceasefire, Ukrainian and Russian officials say.

The ceasefire was due to start in Mariupol at noon local time on Sunday (9pm AEDT) and last for nine hours, the city council of Mariupol said.

A similar plan had to be abandoned on Saturday after the ceasefire was not fully observed, with both sides trading blame.

The head of the military in Ukraine’s separatist-held Donetsk territory said safe passage corridors for residents in Mariupol and nearby Volnovakha will be open again on Sunday.

By night time on Saturday, 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.

Mariupol Mayor Vadym Boichenko said his main priority is to help many of the 400,000 people stuck in the south-eastern city to escape.

“They’re destroying us,” he told Reuters in a video call during which he could hear the sound of explosions from outside.

He said most residents were sleeping in bomb shelters to escape over six days of near-constant bombardment by encircling Russian forces that had cut off food, water, power and heating supplies.

“They’ve been working methodically to make sure the city is blockaded,” 44-year-old Mr Boichenko said.

“They will not even give us an opportunity to count the wounded and the killed because the shelling does not stop.”

A planned evacuation had to be postponed on Saturday after a temporary ceasefire agreed by Moscow and Kyiv was not observed, with both sides trading blame.

Both Mr Boichenko and Ukrainian forces defending Mariupol have also called for military reinforcement, saying Russia will not back down from attempting to seize the city.

For Russia, capture of Mariupol would be a prize – a strategic link between the Russia-backed separatist territories to the north and the land route to the Crimean Peninsula, which Russia annexed in 2014.

-Reuters, with additional reporting by AP