"Tonight, we received a message from the International Red Cross Committee that the Russian Federation confirms its readiness to open access for the humanitarian convoy to the city of Mariupol with transit through the city of Berdiansk," she said in a video posted on Telegram.

"We are sending 45 buses to the Mariupol corridor".

Seventeen buses had already left for Mariupol from Zaporizhzhia, around 220 kilometers (136 miles) to the northwest, she said.

Another 28 were waiting for authorization to pass the Russian checkpoint in Vasylivka, near Zaporizhzhia.

"We will do everything possible to ensure that buses arrive in Mariupol today and pick up people who have not yet been able to get out of the city," Vereschuk said.

So far, with agreements on centralized evacuations not being respected, civilians have only been able to leave Mariupol in their own cars, which is extremely dangerous.

The strategic southern port city has been encircled and shelled by Russian forces since the end of February.

People who have managed to flee and ONGs have described the conditions there as catastrophic, with civilians bunkered down in cellars, deprived of water, food, communication, with dead bodies littering the streets.

Late Wednesday, the Russian defense ministry said that a humanitarian corridor would be opened from Mariupol to Zaporizhzhia, via the Russian-controlled port of Berdiansk, from 10:00 am (0700 GMT) on Thursday.

"For this humanitarian operation to succeed, we propose to carry it out with the direct participation of the UN High Commissioner for Refugees and the International Committee of the Red Cross," the ministry statement said.

The Russian ministry asked Kyiv to guarantee "unconditional respect" for the ceasefire through written notification to the Russian side, the UNHCR, and ICRC before 6:00 am (0300 GMT) Thursday.

Moscow also asked the Ukrainian army to commit to ensuring the security of the bus convoys along the designated corridor.

Deputy premier Vereschuk said that Ukrainian forces had "confirmed they'll guarantee the ceasefire".

Two other evacuation corridors were agreed for Thursday, from Melitopol to Zaporizhzhia and from Energodar, where Ukraine's biggest nuclear plant is situated, also to Zaporizhzhia.