(NewsNation) — Ukraine was, for the third successive day, unable to secure Russia’s agreement to establish any humanitarian corridors to evacuate civilians trapped in cities and towns, Deputy Prime Minister Iryna Vereshchuk said on Tuesday.

She said intensive shelling continued in the Donbas region of eastern Ukraine, where President Volodymyr Zelenskyy says Russia has launched a new offensive.

Efforts to create safe passage for civilians to leave the besieged southern city of Mariupol have failed repeatedly, with each side blaming the other. Russia has denied targeting civilians.

“According to Mariupol: (the) Russians refuse to provide a corridor for the exit of civilians in the direction of Berdyansk,” Vereshchuk said.

Humanitarian corridors are protected paths created for civilians to safely leave dangerous areas or so that aid workers can deliver food, medical supplies, etc., to those who remain. They consist of routes that can be crossed in buses or cars that Russia and Ukraine agree not to attack. They usually link directly to another city under Ukrainian control, like the one that successfully got citizens from Russian-controlled Berdyansk to Zaporizhzhia.

These corridors can also be used to transport food and water into war-torn parts of the country. The United Nations and The Red Cross have repeatedly tried to get aid deliveries into the besieged cities of Mariupol and Kherson, but that has not worked. It even led to an aid truck being captured by Russian forces for several days.

The war isn’t the first time humanitarian corridor have been used; they helped rescue tens of thousands of children trapped in Nazi-controlled areas of Europe in World War II.

Last week, more than 4,000 people used humanitarian corridors to escape Eastern Ukraine on a single day.

Vereshchuk said “difficult negotiations” were taking place to try to arrange humanitarian corridors in the southern region of Kherson and the Kharkiv region of eastern Ukraine.

Vereshchuk also warned Russia on social media that refusing to open humanitarian corridors will justify war crimes trials.