United Nations Secretary General Ban ki-moon has expressed concern over the use of cluster bombs in Yemen by the Saudi-led coalition of Arab nations.

"The Secretary-General (Ban Ki-moon) 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," U.N. spokesman Stephane Dujarric told reporters on Friday, according to UNI news agency.

The U.N chief also warned that the use of prohibited cluster munitions could be war crime. The U.S. backed Saudi coalition has been fighting Iran-backed Shia Houthi rebels since March 2015.

"We have also received troubling reports of the use of cluster munitions in attacks on Sana'a on 6 January in several locations. The use of cluster munitions in populated areas may amount to a war crime due to their indiscriminate nature," Dujarric said, according to Cihan news agency.

Human Rights Watch, in a report, described the cluster bomb attacks by Saudi coalition as a "'war crime" and called for an international inquiry.

"The coalition's repeated use of cluster bombs in the middle of a crowded city suggests an intent to harm civilians, which is a war crime," HRW's arms director Steve Goose said in a statement.

"These outrageous attacks show that the coalition seems less concerned than ever about sparing civilians from war's horrors," he said.

A Saudi-led coalition of the Arab nations resumed airstrikes against Sanaa-based Houthi rebels last week after a two-week ceasefire. The ceasefire agreement between the two sides was announced on Dec. 15 after the resumption of U.N.-mediated Yemen peace talks in Switzerland, as HNGN previously reported.

Nearly 6,000 people - more than half of which were civilians - have been killed in the war-torn country since March 2015.