More than 2.5 million people have fled their homes in Darfur
More than 50,000 people are suffering in the latest cycle of violence in Sudan's West Darfur region, says the UN humanitarian affairs agency, Ocha.
"Some of their homes were burnt to the ground. They have absolutely nothing left," said Ocha's Orla Clinton.
Local residents say government aid raids over the last three weeks have been followed by militia raids.
China's envoy to Darfur has urged Sudan to accept the full deployment of a UN-African Union peacekeeping force.
In a rare public rebuke to Khartoum, visiting envoy Liu Guijin said it should "co-operate better with the international community" on the force, according to a report by Chinese official news agency Xinhua.
The force began deploying in January, but still lacks most of the 26,000 personnel planned for the mission - due in part to Sudanese objections concerning the international composition of the force.
China has come under increasing pressure to use its influence with Sudan to end the fighting.
Ms Clinton said up to an estimated 58,000 civilians in the areas of West Darfur attacked over the last few weeks were particularly vulnerable.
"They are dependent on humanitarian aid, they are at risk of further attacks, and they are in desperate need of further protection," she told the BBC's Network Africa programme.
She said she was particularly concerned about civilians in the Jebel Moun area affected by the latest aerial bombardments.
The Sudanese army lost a helicopter close to the West Darfur capital, Geneina, on Sunday, but the cause of the incident is disputed.
A spokesman for the rebel Justice and Equality Movement (Jem) told Reuters it had downed the helicopter, but the army says the accident was due to a faulty engine.
Meanwhile, the Sudanese defence minister said the army had destroyed nine rebel camps in recent military operations in West Darfur.
Abdel Rahim Mohammed Husein told the official Suna news agency the army now "totally controls" the region including Salia, Jebel Sujuj, Sirba and Jebel Moun.
Mr Liu used unusually frank language
Mr Liu began a five-day visit to Sudan on Sunday, and is set to travel to Darfur on Tuesday, the fifth anniversary of the start of the conflict which has left 200,000 people dead and 2.5 million homeless.
In his comments to Xinhua, Mr Liu said deploying the peacekeeping operation and resolving the Darfur issue required "the joint efforts of all sides.
"First, the Sudan government should co-operate better with the international community and demonstrate greater flexibility on some technical issues. Next, anti-government organisations in the Darfur region should return to the negotiating table."
China has long had strong trade and military links with Khartoum, which is accused of backing militias that have raped and murdered civilians in Darfur - accusations it denies.
But Beijing is keen to show it is playing a positive role in the region, says the BBC's Amber Henshaw in Khartoum.
Mr Liu said Sudan only bought 8% of its weapons from China and said if China stopped selling weapons, they could easily be purchased from other countries.
Steven Spielberg recently pulled out as artistic adviser to the Beijing Olympics, saying China was not doing enough to end the humanitarian crisis in the troubled Sudanese region.