The Sudanese military is said to have renewed its aerial bombing campaign in the west of the Darfur region.
More than 2.5 million people have fled their homes in Darfur
The joint United Nations African Union mission in Sudan, Unamid, said it had received reports of aerial bombings in the Jebel Moun area of the region.
A Unamid spokesman said there was grave concern for the safety of thousands of civilians in the area.
The reports came as China's envoy for Darfur, Liu Guijin, began a five-day visit to the country to push for peace.
China has come under increasing pressure to use its influence with Sudan to end the fighting.
Mr Liu will travel to Darfur on Tuesday, the fifth anniversary of the start of the conflict which has left 200,000 people dead and 2.5m homeless.
'Ready to help'
In a joint news conference with the Sudanese Foreign Minister, Deng Alor, Mr Liu stressed the importance of Darfur to Beijing.
"My message to the media and to the world is that the Chinese government and people are ready to help Sudan and to help the international community to find the solution of the Darfur issue", he said.
The BBC's Amber Henshaw in Khartoum says Beijing is keen to show it is playing a positive role in the region - this week it will provide $11m (£5.6m) of humanitarian assistance.
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.
Activists have accused China of helping to arm pro-Khartoum militia against Darfur's rebel groups, but Mr Liu told the BBC on Friday that only 8% of weapons imported by Sudan came from China in 2006 and insisted it was not fuelling the conflict.
"There are seven countries selling arms to Sudan. So even if China stopped its sale, it still won't solve the problem of arms in Sudan," he said.
Mr Liu is also expected to push Sudan to co-operate on the deployment of more UN-African Union peacekeepers.
The force began deploying in January but it still lacks most of the 26,000 personnel planned for the mission.
The Chinese envoy's visit was announced just days after film director Steven Spielberg 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.
Mr Spielberg said his conscience would not allow him to continue in the role.