Fresh attempts to evacuate civilians from cities under siege in Ukraine are being complicated by constant Russian shelling, Ukrainian officials say.
Humanitarian corridors are being set up from Mariupol, Sumy and towns and villages outside the capital Kyiv.
The situation in Mariupol is particularly desperate after two weeks of bombardment, the UN says, with little access to food, water and power.
The evacuations come as fighting continues around Kyiv.
The BBC's Abdujalil Abdurasulov in Irpin, one of the towns outside the capital being evacuated, says it is not possible to say that the humanitarian ceasefire is holding because explosions and artillery fire, including from the Ukrainian side, can still be heard.
Regional officials also said that fighting in the area was continuing and that there was a constant threat of air attacks.
Mariupol Deputy Mayor Serhiy Orlov told the BBC a convoy had left Zaporizhzhya for the city carrying aid and including buses for the evacuations, but it was not clear whether it would get through.
"This is the seventh attempt. On the previous six it didn't work. The convoys were not let through, they were bombed, the road was mined, there was shelling in the town," he said.
Earlier, Ukraine's Deputy Prime Minister Iryna Vereshchuk said she hoped the day would go well and that "Russia will fulfil its obligations to guarantee the ceasefire".
The UN has said those who remain trapped in Mariupol had become desperate, with basic supplies running out in the city.
"Medicines for life-threatening illnesses are quickly running out, hospitals are only partially functioning, and the food and water are in short supply," the Office for the Co-ordination of Humanitarian Affairs said on Friday.
Speaking to journalists on Saturday, Ukrainian President Volodymyr Zelensky said that about 1,300 of his country's troops had died since the start of the Russian invasion.
Russia invaded Ukraine on 24 February, arguing that it felt threatened by a neighbour intent on joining Western-led organisations such as the Nato military alliance.