Saudi-led airstrikes pounded Shiite rebels and allied forces in Yemen on Friday, killing at least 10 civilians, witnesses said, as the U.N. called for $1.6 billion to help millions of Yemenis avoid a "looming humanitarian catastrophe."

The airstrikes targeted the rebel-held capital, Sanaa, the southern city of Aden and the provinces of Lahj and Jouf early Friday morning, officials and witnesses said. The 10 civilians were killed in strikes on the rebels' northern stronghold, Saada, witnesses said. The officials spoke on condition of anonymity because they were not authorized to speak to the press. The witnesses insisted on anonymity for fear of retribution.

The fighting in Yemen pits Shiite rebels, known as Houthis, and troops loyal to former President Ali Abdullah Saleh against southern separatists, local and tribal militias, Sunni Islamic militants and loyalists of internationally recognized President Abed Rabbo Mansour Hadi, who is in exile in Saudi Arabia. The rebels seized the capital in September.

The U.S.-backed coalition began launching airstrikes against the Houthis and their allies in late March, but has made little progress in pushing the rebels back.

U.N. Under-Secretary-General for Humanitarian Affairs Stephen O'Brien on Friday warned of a "looming humanitarian catastrophe," with at least 11.7 million people affected by the conflict. He requested $1.6 billion dollars in emergency aid.

"People across the country are struggling to feed their families. Basic services are collapsing in all regions. Millions of families no longer have access to clean water, proper sanitation or basic health care. Deadly diseases such as dengue and malaria have broken out, and supplies for acute trauma care are running dangerously low," he said in Geneva.

More than 1,000 civilians have been killed by the fighting and airstrikes, and a million have fled their homes.

In Geneva, U.N. special envoy Ismail Ould Cheikh Ahmed was shuttling between delegations from the Hadi government and the Houthis on Friday, hoping to broker an agreement on a humanitarian truce, U.N. spokesman Ahmad Fawzi said. The exiled government delegation is expected to depart Geneva on Saturday evening and the rebel delegation on Sunday.

Yemen's Foreign Minister Riad Yassin told the Al-Jazeera network that international partners are "frustrated" and said the Houthis would bear the blame if the talks failed.

The government has insisted on the implementation of a Security Council resolution demanding the withdrawal of the Houthis and their allies from captured cities, including the capital. The Houthis want a temporary cease-fire, which the government has rejected, saying the rebels exploited an earlier humanitarian pause to grab more territory.