Ukrainian civilians began leaving two besieged areas on Tuesday after Russia opened "humanitarian corridors" for them, but Kyiv said Russian forces had shelled an evacuation route from the port city of Mariupol.

Soon after residents began leaving the northeastern city of Sumy and the town of Irpin near the capital Kyiv, foreign ministry spokesperson Oleg Nikolenko reported shelling near Mariupol.

"Ceasefire violated! Russian forces are now shelling the humanitarian corridor from (nearby) Zaporizhzhia to Mariupol," Nikolenko wrote on Twitter.

"8 trucks + 30 buses ready to deliver humanitarian aid to Mariupol and to evac (evacuate) civilians to (nearby) Zaporizhzhia. Pressure on Russia MUST step up to make it uphold its commitments."

There were no immediate reports of any casualties and Nikolenko gave no other details. Russia did not immediately comment on Nikolenko's remarks.

Civilians have been trapped by fighting since Russian troops invaded Ukraine on Feb. 24, and officials say food, water and other supplies have been running low in some cities.

Humanitarian corridors from Mariupol failed on Saturday and Sunday, with each side accusing the other of continuing to fire.

But Russia's defence ministry said on Tuesday corridors had been opened to evacuate people from Kyiv, Chernihiv, Sumy, Kharkiv and Mariupol, and that Russian forces had introduced a "silent regime" from 0700 GMT, Interfax news agency reported.

"We have already started the evacuation of civilians from Sumy to Poltava (in central Ukraine), including foreign students," Ukraine's foreign ministry said in a tweet.

Dmytro Zhyvytsky, the governor of the Sumy region, said in televised comments around noon (1000 GMT) that the evacuation from Sumy was continuing and that the temporary ceasefire had largely held there.

The first wave of vehicles that left the city met around 160 Russian military vehicles coming towards them but the incident ended peacefully when the civilian convoy stopped to let the Russian forces pass, he said.

Around 1,000 foreign students had already been evacuated, Zhyvytsky said, and convoys of 20-30 private cars were leaving in waves.

Oleksiy Kuleba, governor of the Kyiv region, said more than 150 people had been evacuated from Irpin by 0730 GMT.

FLOOD OF REFUGEES

People trying to flee Irpin on Sunday were caught in Russian shelling, Reuters witnesses said, and a former mayor warned of a "humanitarian catastrophe" in the area.

Authorities in the Sumy region said 21 civilians, including two children, had been killed in a Russian air strike on a residential street in Sumy late on Monday. Reuters was unable to verify the report.

President Volodymyr Zelenskiy said a child had died of dehydration in Mariupol.

The child's death could not immediately be confirmed independently. Moscow calls its actions in Ukraine a "special military operation" to disarm its neighbour and arrest leaders it calls "neo-Nazis". It denies targeting civilians.

Some 2 million refugees have already fled Ukraine to other countries, Filippo Grandi, the head of the United Nations refugee agency, said