A Ukrainian official says a second attempt to evacuate civilians from a southern city under siege for a week has failed due to continued Russian shelling.

Most people in the port city of Mariupol are sleeping in bomb shelters to escape over six days of near-constant bombardment by encircling Russian forces that has cut off food, water, power and heating supplies, according to the Ukrainian authorities.

Evacuations were scheduled to begin at noon local time during a 10am to 9pm local ceasefire, Ukrainian military authorities said earlier on Sunday.

But Interior Ministry adviser Anton Gerashchenko said the planned evacuations along designated humanitarian corridors were halted because of an ongoing assault.

The International Committee of the Red Cross said the evacuation effort had failed after "hostilities resumed".

"We remain in Mariupol and are ready to help facilitate further attempts – if the parties reach an agreement, which is for them alone to implement and respect," it said on Twitter.

A day earlier, Ukrainian officials similarly said Russian artillery fire and air strikes had prevented residents from leaving before the planned evacuations got underway in Mariupol and the nearby city of Volnovakha.

Then, Mr Putin accused Ukraine of sabotaging the effort.

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

"They're destroying us. They've been working methodically to make sure the city is blockaded," Mr Boichenko said.

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

Shelling from the Russian side destroyed half of a convoy of buses Mr Boichenko's team had readied for the evacuation, he said.

"They lied to us, what's more, the moment when people were trying to get out to go to these corridors, the shelling started again," he said, describing residents' fear and anger at having to flee back to the shelters on Saturday.

The civilian death toll from hostilities across Ukraine since Moscow launched its invasion stood at 364, including more than 20 children, according to the United Nations on Sunday, with hundreds more injured.

Moscow has repeatedly denied attacking civilian areas.

US has 'very credible reports' of Russian attacks on civilians

Meanwhile, US Secretary of State Anthony Blinken said the US had seen "very credible reports" of deliberate Russian attacks on civilians and that Washington was documenting these reports to support appropriate organisations in their potential war crimes investigation.

"We've seen very credible reports of deliberate attacks on civilians which would constitute a war crime," Mr Blinken told CNN.

"We've seen very credible reports about the use of certain weapons.

"What we're doing right now is documenting all of this, putting this all together, looking at it and making sure that as people and the appropriate organisations and institutions investigate whether war crimes have been or are being committed, that we can support whatever they are doing."

In an interview on CNN, European Commission President Ursula von der Leyen said an investigation was needed into whether Russia was committing war crimes in Ukraine.

Putin's invasion 'going according to plan'

The news came as Mr Putin said on Sunday that his campaign in Ukraine was going according to plan and would not end until Kyiv stopped fighting.

He made the comments in a phone call with Turkish President Tayyip Erdogan, who appealed for a ceasefire in the conflict that the UN said had created the fastest-growing refugee crisis in Europe since World War II.

Mr Putin told Mr Erdogan he was ready for dialogue with Ukraine and foreign partners but any attempt to draw out negotiations would fail, a Kremlin statement said.

Russian television said Mr Putin also held talks on Sunday with French President Emmanuel Macron, who has remained in regular contact but has yet to convince Moscow to call off a campaign now into the 11th day.

The French leader told Mr Putin of his concerns about a possible imminent attack on the Ukrainian city of Odessa, according to a statement from Mr Macron's office.

Mr Macron also stated the importance of finding a negotiated settlement to the Ukraine crisis, and of protecting Ukraine's nuclear facilities.

Kyiv renewed its appeal to the West to toughen sanctions beyond existing efforts that have hammered Russia's economy.

It also requested more weapons, including a plea for Russian-made planes, to help it repel Russian forces.

Mr Blinken said Washington was "very, very actively" considering how it could backfill aircraft for Poland, if Warsaw decided to supply its warplanes to Ukraine, speaking on a trip to neighbouring Moldova.

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AP/Reuters