The United Nations representative in Yemen has said that the Saudi-led coalition is bombing "effectively, trapped civilians".
Civilians in the northern city of Saada are struggling to flee Saudi-led coalition air strikes targeting Houthi rebels, reports and aid workers say.
The UN also warned that the indiscriminate bombing of populated areas is against international law.
Air strikes have killed at least 1,400, more than half civilians, the UN says.
Air strikes also targeted the home of former President Ali Abdullah Saleh in the capital, Sanaa, early on Sunday. The former president is said to be unharmed.
Mr Saleh is allied with the Houthi rebels - who overran Sanaa from their base in north Yemen last year and now control much of the country.
The UN humanitarian co-ordinator for Yemen, Johannes Van Der Klauuw, said he was "deeply concerned" by the impact of the latest strikes on the north.
"Many civilians are effectively trapped in Saada" because of fuel shortages, he said.
The Saudi-led coalition says it regards Saada a "military zone". On Friday it dropped leaflets warning residents to leave.
Mr Van Der Klauuw said that the decision to target the entire province "will put countless civilians at risk".
Teresa Sancristoval, who works for the medical charity MSF at Saada's hospital, described intense bombing on Friday night, with reports of up to 140 strikes hitting the city in total.
She said her team had been treating women who were in labour, but five of them fled because of the intensity of the air strikes.
The Saudis have accused the Houthi rebels of hiding troops and weapons in civilian areas to prevent them being targeted by air strikes.
Saudi Arabia has offered a five-day truce starting on Tuesday to allow the delivery of humanitarian supplies - but warned that it would respond to any violations.
On Sunday renegade Yemeni troops allied to the Houthis accepted the proposal "following mediation from friendly countries" - however, it is unclear whether they were speaking on behalf of the rebel movement as a whole.
Earlier senior Houthi official Mohamed al-Bukhaiti told the BBC that the ceasefire had not been formally proposed and the Houthis would not respond until a proper plan was laid out.
Sunni Muslim Saudi Arabia has accused Shia rival Iran of arming the Houthis, a charge Iran and the Houthis deny.
The Saudi-led coalition aims to restore the government of exiled President Abdrabbuh Mansour Hadi.
He fled the Sanaa in February and later left the country for Saudi Arabia.
Houthis - The Zaidi Shia Muslim rebels from the north overran Sanaa last year and then expanded their control. They want to replace Mr Hadi, whose government they say is corrupt. The US alleges Iran is providing military assistance to the rebels.
Ali Abdullah Saleh - Military units loyal to the former president - forced to hand over power in 2011 after mass protests - are fighting alongside the Houthis.
Abdrabbuh Mansour Hadi - The president fled abroad in March as the rebels advanced on Aden, where he had taken refuge in February. Loyal soldiers, Sunni Muslim tribesmen and Southern separatists have formed militia to fight the rebels.
Saudi-led coalition - A US-backed coalition of nine, mostly Sunni Arab states says it is seeking to "defend the legitimate government" of Mr Hadi.
Al-Qaeda in the Arabian Peninsula - AQAP opposes both the Houthis and President Hadi. A rival affiliate of Islamic State has also recently emerged.