Russia announced yet another ceasefire and a handful of humanitarian corridors to enable civilians to flee Ukraine starting Monday, but previous such measures have fallen apart and Moscow's armed forces continued to pummel some Ukrainian cities with rockets even after the announcement.

A day earlier, hundreds of thousands of civilians attempting to flee to safety were forced to shelter from what Ukrainian officials said was Russian shelling in cities in the centre, north and south.

On Sunday the second attempt to evacuate civilians from the southern city of Mariupol failed due to the continued Russian attacks, Ukrainian Interior Ministry adviser Anton Gerashchenko said. The port city has been under siege for a week.

"There can be no 'green corridors' because only the sick brain of the Russians decides when to start shooting and at whom," he said on Telegram.

The city council in Ukraine's port city of Mariupol said it will begin efforts to evacuate its civilian population after Saturday's attempts were scuppered by cease-fire violations.

Ahead of the third round of talks planned for Monday, the Russian Defense Ministry said a ceasefire would start in the morning, and safe passages would open for civilians from the capital of Kyiv, the southern port city of Mariupol, and the cities of Kharkiv and Sumy. Some of the evacuation routes, however, would funnel civilians toward Russia or its ally Belarus - unlikely destinations for many Ukrainians who would prefer to head toward countries on the western and southern borders.

A senior Ukrainian official rejected those proposals.

Many residents are trapped in cities  without water and food

It wasn't immediately clear if fighting would stop beyond the areas mentioned or when the cease-fire would end. Hopes were dim that the latest round of talks would yield any breakthroughs.

Some of the 400,000 residents trapped by Russian forces were set to start evacuating at 12 p.m. local time (10 a.m. GMT) on Sunday under a temporary cease-fire that was to last until 9 p.m. About 440,000 people resided in the port city on the Sea of Azov before the Russian invasion. Many residents still there are without electricity, water and heat after days of Russian bombardment.

A similar plan had to be abandoned on Saturday after the cease-fire was not fully observed, with both sides trading blame. It was not clear how many civilians were able to escape during the brief window when the fighting paused. Buses had been arranged to transport people out of the besieged city.