Yemen's Houthi rebels pounded an area in the southern province of Taez shortly after the start early Monday of a humanitarian truce declared by the Saudi-led coalition bombing the Iran-backed insurgents.
The rebels shelled residential areas in Jebel Sabr, witnesses said, although in the capital Sanaa and in central Yemen, officials and residents said the ceasefire appeared to hold.
The Saudi-led coalition backing loyalist troops on Saturday announced the five-day truce from midnight on Monday (Sunday 2100 GMT) to allow aid deliveries, but also said it reserved the right to respond to "military activity or movement".
Mohammed Ali al-Houthi, the self-described "president of the High Committee of the Revolution", a body formed by Houthi militants, said in comments published by the rebel-controlled Saba news agency that his group had not been consulted by the UN.
The group could therefore not give a "negative or positive" answer about the truce, he said.
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 growing number of civilian casualties... in the unfolding humanitarian catastrophe make a pause and an eventual extension an imperative," he said in a statement issued late Sunday.
Impoverished Yemen has been rocked by months of fighting between Houthi Shia rebels and troops loyal to exiled President Abd-Rabbou Mansour Hadi, supported by the Saudi-led Arab coalition.
Pro-government forces battled retreating rebels on the northern outskirts of Yemen's second city Aden Sunday, with the coalition pounding the insurgents' positions there right up to 15 minutes before the truce.
Barely an hour after the ceasefire came into force, sporadic gunfire rang out just north of the southern port city, an AFP correspondent said.
Pro-Hadi Popular Resistance militiamen had attacked the Houthis overnight Saturday on the northern outskirts of Aden, forcing the rebels out of the Basateen and Jawala areas.
The loyalist forces have been bolstered by new weaponry and armoured vehicles delivered by the coalition.
They also benefited from coalition air support, military sources said, adding that dozens of rebels were killed in the latest fighting.
Seven pro-Hadi fighters were also killed and 29 were wounded, a medical source said.
Further north, troops loyal to Hadi forced rebels out of the town of Sabr in Lahj province, General Fadhel Hassan told AFP.
Hassan said pro-Hadi troops had taken the town that links Aden to Huta, the provincial capital of Lahj, adding that Huta is the next target before reaching Al-Anad, the country's largest airbase.
The strategically important base housed US troops involved in a long-running drone war against Al-Qaeda before the fighting forced them to withdraw.
Coalition air strikes killed 17 rebels in Lahj on Sunday and 14 in Abyan province, loyalist military sources said.
In a sudden turn of events, pro-Hadi forces last week regained control of much of Aden, which Houthi rebels overran in March.
Troops trained and armed by the coalition appeared to have triggered the shift in the balance in the Hadi loyalists' favour.
The Houthis and allied renegade forces loyal to former president Ali Abdullah Saleh advanced on Aden after Hadi took refuge in the city following his escape from house arrest in Sanaa in February.
He later fled to Saudi Arabia, which assembled an Arab coalition that began an air campaign in late March against the rebels in a bid to restore the UN-backed leader.
In Riyadh, Hadi on Sunday received the UN envoy to Yemen, Ismail Ould Cheikh Ahmed.
The two "discussed coordination over humanitarian aid delivery within the framework of the declared truce", a Yemeni presidency source said.
A UN-declared six-day truce failed to take hold earlier this month after it was ignored by both the coalition and the rebels.
The coalition said at the time it did not receive a request to halt operations from Hadi.
But desperately needed relief supplies have recently begun to trickle into Aden after pro-Hadi fighters secured the city.
A ship carrying 3,000 tonnes of supplies from the UN's World Food Programme (WFP) docked in Aden on Tuesday, the first UN vessel to reach the city in four months of fighting.
Other ships from the UN and Gulf countries followed.
On Saturday, a WFP ship carrying 3,400 tonnes of mixed food supplies -- enough to feed 192,000 people for a month -- arrived in Aden, WFP spokeswoman Reem Nada told AFP.
On Sunday, a Saudi vessel loaded with 4,000 tonnes of food aid docked at Aden port at midday, an AFP correspondent said.
The United Nations says the conflict has killed more than 3,640 people, around half of them civilians, since late March.
On Friday, the International Committee of the Red Cross warned that civilian suffering in Yemen had reached "unprecedented levels".