ADEN (AFP) - Scattered clashes between rebels and pro-government forces undermined a fragile ceasefire in Yemen on Thursday (Oct 20) as global pressure intensified for a lasting truce in a country where millions are homeless and hungry.

At least 11 people were killed on the first day of the UN-brokered ceasefire, despite all parties insisting they will pause the fighting.

The truce, for an initial period of three days, took effect shortly before midnight on Wednesday to allow sorely needed aid deliveries.

A Saudi-led Arab coalition intervened in March 2015 to support the government of President Abedrabbo Mansour Hadi after Huthi rebels overran much of the impoverished country on the southern tip of the Arabian Peninsula.

Saudi Arabia and Washington accuse Iran of arming the insurgents allied with forces loyal to ex-president Ali Abdullah Saleh, charges Tehran denies, and the conflict has stoked tensions between the rival Middle East powers.

The coalition and Hadi both vowed to abide by the ceasefire despite alleged violations by the rebels.

Pro-government forces declared "respect for the truce" but reserved the right to respond to infractions.

The rebels said they would respect the ceasefire as long as "the enemy" abides by it, but urged their fighters to be ready to retaliate against "all aggression." The coalition has said it will continue an air and maritime embargo, to prevent weapons shipments to the rebels, and will maintain airborne reconnaissance.

At dawn, coalition aircraft flew twice over the rebel-held capital Sanaa without conducting raids, according to residents, who said they have had enough of failed truces.

"They're mocking us with a three-day truce," said Ali al-Doush, a 32-year-old civil servant who has not been paid for three months.

"We want an end to the war."

Nearly 6,900 people have been killed - more than half of them civilians - in the conflict, while another three million are displaced and millions more need food aid.

The deaths on Thursday included three civilians killed in a coalition air raid on Saada province, a historic Huthi stronghold, according to rebel-controlled media.

Five pro-government fighters were killed in two rebel attacks in Saada and Hajja provinces, according to loyalist forces, while three rebels were said to have died in attacks in the Red Sea province of Hodeida.

A man and his daughter were also wounded in a cross-border attack from Yemen on the kingdom's Jazan region, according to Saudi civil defence authorities.

Military sources and residents said there had also been fighting around besieged third city Taez, and pro-Hadi positions came under fire in Sarwah, east of Sanaa.

EU foreign policy chief Federica Mogherini late Wednesday said the truce should be a first step towards resuming UN-led peace talks, urging warring parties to respect it.

The ceasefire will allow urgent humanitarian assistance to reach large parts of the population that have suffered drastic shortages, she said.

The last ceasefire attempt began in April and collapsed as UN-brokered peace talks broke down in August.

Fighting then escalated, until an October 8 coalition air strike which the UN said killed more than 140 people and wounded at least 525 at a funeral in Sanaa.

The United States announced an "immediate review" of its intelligence and refuelling assistance to the coalition, whose investigative team said the funeral was "wrongly targeted".

The US Navy also for the first time targeted rebels directly earlier this month, hitting radar sites which Washington said were involved in missile launches against a US warship and other vessels.

The UN's humanitarian coordinator for Yemen, Jamie McGoldrick, on Wednesday told reporters he hoped the truce will provide a chance for aid workers to reach areas isolated by the fighting.

"We're seeing very high levels of malnutrition and food insecurity," he said.

The ceasefire does not apply to jihadists who have exploited the conflict to gain ground in the south.

Suspected Al-Qaeda militants on Thursday killed four soldiers at a checkpoint in the southern province of Abyan, a security official said. One attacker was killed and another wounded.