Warplanes from the Saudi-led coalition raided positions of the Iran-backed Shiite insurgents east of the rebel-held capital Friday, military sources said.

The air strikes targeted Huthi rebels in the west of Marib province, amid ongoing clashes between them and forces loyal to President Abedrabbo Mansour Hadi.

Pro-Hadi fighters backed by Gulf forces have been pushing to regain control of Marib province in an apparent bid to advance on Sanaa.

They set their sights on Marib after pushing the rebels out of five southern provinces, including the port city of Aden to which the government returned last month after six months in exile.

Raids also targeted rebels in Hodeida in western Yemen, other military sources said.

On Friday evening, coalition warplanes struck rebel arms depots on the Nahdain hill south of Sanaa, witnesses said.

After they overran Sanaa unopposed in September 2014, the rebels widened their control to several Yemeni provinces, advancing in March on Aden where Hadi had taken refuge before fleeing to Riyadh.

The coalition then launched a fierce air campaign against them and allies loyal to ousted president Ali Abdullah Saleh.

The coalition has come under mounting criticism over the civilian death toll in its campaign.

On Thursday, the UN under secretary general for humanitarian affairs, Stephen O'Brien, called for a swift investigation of a suspected deadly air strike on a wedding in Dhamar province the day before.

Medical sources said at least 28 people were killed, while the rebel-controlled Saba news agency said 51 died.

The coalition has categorically denied any bombing there, just as it did of a wedding near Mokha last month at which 131 people were killed.

"We did not conduct any operation in Dhamar," coalition spokesman Brigadier General Ahmed al-Assiri said Thursday.

"Targeting remains difficult in the current environment... lack of intelligence and deliberate acts of perfidy by Huthis and loyalists," said Andreas Krieg, a professor at the Department of Defence Studies at King's College London.

"I think that these attacks were committed by Huthi rebels or loyalists trying to intimidate local communities. Locals tend to think that they were targeted by air strikes," said Krieg, who also serves as a consultant to the Qatari armed forces.