U.N. Secretary-General Antonio Guterres appealed on Tuesday for a halt in fighting in Ukraine during Orthodox Christians' Holy Week, invoking Easter pleading for a four-day "pause" to allow humanitarian aid and evacuations.

Noting that the Easter season is coming amid an intensifying Russian offensive in eastern Ukraine, the U.N. chief said the need for a cease-fire is all the more urgent.

"The onslaught and terrible toll on civilians we have seen so far could pale compared to the horror that lies ahead. This cannot be allowed to happen," he told reporters, urging Russians and Ukrainians "to silence the guns and forge a path to safety for so many at immediate risk."

He suggested the pause begin Thursday, which is Holy Thursday for Orthodox Christians, who make up most of Ukraine's population.

Guterres said the goal is to open "humanitarian corridors" so civilians can escape embattled areas and more humanitarian convoys can get in, particularly to such areas as Donetsk, Luhansk, Kherson and Mariupol. More than 4 million people in those areas, and 12 million nationwide, need food, water and medicine.

The U.N. humanitarian chief, Martin Griffiths, had traveled to Ukraine and Russia earlier this month to try to gauge the potential for a cease-fire and emerged saying he wasn't optimistic.

But Griffiths suggested Monday there could be "some ripeness" for a cease-fire as the Orthodox Easter holiday approaches Sunday. He broached the idea Tuesday with the Ukrainian Council of Churches and Religious Organizations, an interfaith group, Guterres said.

"The four-day Easter period should be a moment to unite around saving lives and furthering dialogue to end the suffering in Ukraine," Guterres said.

The proposal comes after the U.N. recently helped foster a two-month truce in Yemen's civil war. The truce took effect as the Islamic holy month of Ramadan began.

The U.N. let Russia and Ukraine know before Guterres went public with his appeal Tuesday, spokesperson Stephane Dujarric said. As for its prospects, "We are always hopeful," he said.

The U.N. Security Council was due to meet later Tuesday to discuss the war, particularly the millions of people it has displaced.