Three evacuation corridors are planned to be opened in Ukraine's northeastern Sumy region on Thursday, the head of the regional administration Dmytro Zhyvytsky said on his official Telegram channel.

"Green corridors" allow civilians to escape to safer regions of the country as Russian forces bombard Ukrainian cities.

The proposed evacuation routes were tentatively set to start at 9 a.m. local time (2 a.m. ET) Thursday, he said, with civilians heading to Poltava, about 160 kilometers (100 miles) away in central Ukraine.

Efforts to evacuate civilians from Sumy have been stepped up after at least 21 people were killed Tuesday in a Russian airstrike on the city.

Some context: Nearly 35,000 people were rescued via humanitarian corridors in Ukraine on Wednesday, according to President Volodymyr Zelensky. Ukrainian authorities were preparing six humanitarian corridors to get people out of areas under attack by Russian forces, he added.

But local authorities in areas close to Kyiv said efforts to evacuate people to safety Wednesday failed. The city council of Bucha said 50 buses were blocked by the Russian military in nearby Stoyanka.

A loud and growing chorus is calling for an international effort to investigate potential war crimes committed by Russian forces in Ukraine.

Ukrainian President Volodymyr Zelensky called the Russian bombing of a maternity and children's hospital in the southern city of Mariupol Wednesday a "war crime" and "proof of a genocide of Ukrainians."

"We never did and we would have never done nothing like this anything close to this war crime in any cities of Donetsk or Luhansk region, or any region," he said, referring to two separatist-held territories of eastern Ukraine.

The UN said it would follow up "urgently" and that health care facilities, hospitals and health workers should not "ever, ever be a target."

On Wednesday, Ukrainian Prosecutor General Iryna Venediktova called for a "new model" of investigative efforts to tackle alleged war crimes in the country, saying a special tribunal should be created for the "unprecedented" crisis. She said 39 countries have joined Ukraine’s appeal to the International Court of Justice (ICJ) against Russia.

Russia's attack of a nuclear power plant, suspected use of cluster bombs and so-called vacuum bombs in dense areas, and targeting of hospitals, schools and civilian areas have also been described as war crimes.

Last week, the International Criminal Court (ICC) said it would immediately proceed with an active investigation of possible war crimes following Russia’s invasion of Ukraine. The UN is also establishing a Commission of Inquiry to investigate possible human rights violations by Russia.

What is a war crime? The ICC has specific definitions for genocide, war crimes, crimes against humanity and the crime of aggression. Targeting civilian populations, violating the Geneva Conventions, targeting specific groups of people and more could be potential Russian war crimes. "One thing is certain, that intentionally directing shelling or targeting civilians or civilian objects is a crime within the jurisdiction of the court," the ICC’s chief prosecutor, Karim Khan, told CNN last week.

What is the ICC? Located in The Hague, Netherlands, and created by a treaty called the Rome Statute first brought before the UN, the ICC operates independently. Some 123 countries are parties to the treaty, but there are very large and notable exceptions, including Russia and the United States. And, for that matter, Ukraine. (CNN)