Russia said it would start a temporary cease-fire and humanitarian corridor out of Mariupol from 10 a.m. Tuesday.- However,
Ukraine said Tuesday that Russian forces started shelling the zone.
Ukraine said Russian forces shelled a humanitarian corridor it promised not to attack on Tuesday.
Russia told the Red Cross it would open a humanitarian corridor between the southern port city of Mariupol and southeastern city of Zaporizhzhia "for the evacuation of people and the transportation of humanitarian cargo," Ukraine's intelligence service tweeted Tuesday.
The Russian defense ministry confirmed the corridor would be open as of 10 a.m. local time on Tuesday, the state-run Tass news agency reported.
The rules of a humanitarian corridor is that neither side's forces can attack during that period.
Ukraine's deputy prime minister Iryna Vereshchuk said in a video address earlier Tuesday that 30 buses were headed to Mariupol to collect civilians trapped there.
But Ukraine's foreign ministry tweeted shortly after that "Russian forces are now shelling the humanitarian corridor from Zaporizhzhia to Mariupol."
Ukraine's military wrote in a Facebook post that "the invaders did not let children, women, elderly people out of the city. The enemy started the attack exactly in the direction of the humanitarian corridor."
There were no reports of injuries or deaths at time of publication. Russia has not publicly commented on the accusations.
Mariupol, a strategic port city on the Azov Sea, has been besieged and shelled by Russian forces for almost a week. The city is located between the Donbas and Crimea, which Russia annexed in 2014, and taking control of it would allow pro-Russian forces from those regions to join up.
Sergei Orlov, the city's deputy mayor, told the BBC's "Today" program in an interview aired Tuesday that his city had neither water nor heating, and was running out of food.
Ukraine and Russia also agreed on a humanitarian corridor out of the northeastern city of Sumy earlier Tuesday.
The Sumy corridor, which would run to central Ukraine, would stay open from 10 a.m. until 9 p.m. local time on Tuesday, Ukraine's foreign ministry tweeted. The Russian defense confirmed the agreement.
At least 1,200 international students are among those trapped in the city, The Guardian reported.
Mykhailo Ananchenko, the governor of the Sumy region, wrote on Facebook that there were "sounds of tanks" near the corridor, but said in a follow-up post that the convoy was moving slowly.
"Gradually the column starts movement, no shooting! The movement has been restored!" he wrote.
Ukraine has for days tried to negotiate with Russia for multiple humanitarian corridors out of the country. Ukrainian officials, including President Volodymyr Zelensky, have accused Russian forces of shelling civilians.