The U.N. secretary-general expressed deep concern Friday at the "intensification" of airstrikes by the Saudi-led, U.S.-supported coalition in Yemen and warned that the reported use of cluster bombs in populated areas could amount to a war crime.

A statement from Ban Ki-moon's spokesman came a day after the U.N. chief condemned Yemen's expulsion of the U.N.'s human rights representative in the country. The U.N. human rights office said this week it received allegations that the Saudi-led forces used cluster bombs.

Ban has received "troubling reports" of the use of cluster munitions in attacks Wednesday on several locations in Yemen's capital, Sanaa, spokesman Stephane Dujarric said Friday.

Dujarric said the secretary-general is particularly concerned about reports of "intense airstrikes in residential areas and on civilian buildings in Sanaa, including the Chamber of Commerce, a wedding hall and a center for the blind."

"The use of cluster munitions in populated areas may amount to a war crime due to their indiscriminate nature," Dujarric said, adding that international human rights law and international humanitarian law prohibit attacks directed against civilians and civilian infrastructure.

Yemen's conflict pits the government, backed by the Saudi-led coalition, against Shiite rebels known as Houthis allied with a former president and backed by Iran. The Houthis took over Sanaa in September 2014, and the Saudi-led coalition began airstrikes against the Houthis in March.

Peace negotiations were launched in December and a cease-fire was declared, but both government forces and rebels ignored it.

The truce formally ended last weekend, just as the Saudis broke diplomatic ties with Iran following attacks by Iranian protesters on its diplomatic missions in response to the Saudi execution of a Shiite cleric.

The Saudi-Iran rupture has raised concerns about peace prospects in Yemen, the Arab world's poorest country, where the U.N. says millions of people are in need of basics like food and fuel.

The U.N. special envoy for Yemen, Ismail Ould Cheikh Ahmed, is trying to get commitments from both sides for a new cease-fire and talks. He was in the Saudi capital, Riyadh, on Friday and will be going to Yemen soon, Dujarric said.