A humanitarian truce in Yemen has failed to take hold after Saudi-led warplanes Tuesday resumed raids on rebels who clashed with loyalists, military sources said.

Iran, which backs the Shiite Huthi rebels, accused its Sunni regional rival Riyadh of "playing with fire and with the ceasefire".

An international rights group, meanwhile, urged the United Nations to investigate air strikes last week that killed scores of civilians, calling them an "apparent war crime".

The five-day truce that began on Monday was unilaterally declared by the pro-government Arab coalition led by Saudi Arabia to allow the delivery of desperately needed relief supplies.

But the coalition, which began its air campaign against the Shiite Huthi rebels and their allies in late March, had said it also reserved the right to strike against any military movement by the insurgents.

Air strikes targeted rebels north of Aden, Yemen's second city, most of which was recaptured by loyalists last week after four months of ferocious fighting, military sources said.

Raids also struck rebels in nearby Lahj province, where forces loyal to exiled President Abedrabbo Mansour Hadi have tightened the noose on insurgents in the strategic Al-Anad airbase.

Three air strikes targeted a rebel convoy near Sabr, north of Aden, where loyalists had advanced over the past days, military sources said.

- Saudis 'playing with fire' -

Other raids hit a building occupied by insurgents in Jaawala -- also north of Aden -- and an overnight strike hit rebels in Marib east of Sanaa, witnesses said.

Fighting around Aden has killed 28 people since Monday, among them 22 rebels, two civilians and two pro-Hadi fighters, city health chief Al-Khader Laswar said.

On Monday, coalition warplanes mistakenly hit positions of pro-government forces in Lahj, killing 12 people and wounding 30, military sources and witnesses said.

Iran's deputy foreign minister, Hossein Amir-Abdollahian, accused Riyadh Tuesday of stirring regional unrest.

"Saudi Arabia, by committing a strategic error in attacking Yemen, is playing with fire and today is playing with the ceasefire," Iranian state television quoted him as saying.

"The continued use of force in Yemen highlights the lack of security and promotes terrorism in the region," Amir-Abdollahian said.

Human Rights Watch, meanwhile, called on the United Nations to investigate coalition air strikes that hit residential areas, condemning them as an "apparent war crime".

Last week, coalition aircraft repeatedly struck two compounds housing the families of workers at the Mokha Steam Power Plant, killing 65 civilians, the New York-based watchdog said.

"With no evident military target, this attack appears to be a war crime," said HRW senior emergencies researcher Ole Solvang.

- Attacks on humanitarian workers -

Ahead of the unilateral ceasefire, which took effect from midnight Sunday, UN chief Ban Ki-moon made a plea for all sides to "agree to and maintain the humanitarian pause for the sake of all the Yemeni people".

The rebels said they were not consulted about the truce.

The UN's aid chief Stephen O'Brien, meanwhile, urged the Security Council to redouble efforts to secure a pause in the conflict, which he said had "brought appalling damage on an already suffering people".

Four months of fighting in Yemen have left 3,984 people dead, nearly half of them civilians, according to the United Nations.

The International Committee of the Red Cross urged the warring parties Tuesday to respect the safety of humanitarian workers, after two people were killed when a Red Crescent ambulance came under fire in the third city of Taez.

"It is extremely worrying that the past couple of months have seen an increasing number of attacks against health-care workers and relief personnel in the country," a statement said.

Johannes Van Der Klaauw, the UN humanitarian coordinator for Yemen, made an urgent appeal Monday for humanitarian access, describing Adan "devastated" after a visit to the city.

"I repeat my plea to all parties of the conflict to put an end to the attacks on civilians and to end the destruction of critical infrastructure, vital for supplying essential goods and services to the civilian population," he said.

"The UN and its partners are committed to rapidly scaling up the humanitarian response effort across the whole of Yemen."