U.N. Secretary-General Antonio Guterres said Thursday that a third evacuation operation is underway to assist civilians in Ukraine’s southern city of Mariupol and the Azovstal steel plant.

“We must continue to do all we can to get people out of these hellscapes,” he told the U.N. Security Council, refusing to go into details about the sensitive mission.

The U.N. chief met Russian President Vladimir Putin in Moscow on April 26 and then traveled to Kyiv to see Ukrainian President Volodymyr Zelenskyy on April 28.

During his meeting in Moscow, the Russian president agreed to allow some evacuations from the besieged southern city of Mariupol and the mammoth steel plant where hundreds of civilians have been sheltering throughout the 10-week war.

This week, 101 civilians from inside Azovstal and around 400 others from nearby areas were safely evacuated by a joint U.N.-International Committee of the Red Cross team.

“I hope that continued coordination with Moscow and Kyiv will lead to more humanitarian pauses to allow civilians safe passage from the fighting and aid to reach those in critical need,” Guterres told council members.