By Karen Allen
BBC News, west Darfur
The UN's new humanitarian chief, John Holmes, has spoken of the fragile morale among relief workers in Sudan's troubled region of Darfur.
Many people in the region say they feel insecure and cut off
Mr Holmes warned it could take just one major security incident to prompt a "humanitarian collapse", endangering tens of thousands of refugees.
The four-year conflict between rebel groups and pro-government Arab militia has seen more than 2.5m displaced.
Mr Holmes is on a tour of Sudan, Chad and the Central African Republic.
Crowds of children chanted their welcome to the UN's new humanitarian chief as he arrived in the rebel stronghold of Jebel Marra, west Darfur.
It was a very different scene to Saturday when Mr Holmes was barred from entering a camp for displaced people in north Darfur by the Sudanese military, though they later apologised.
But the message from some of the tens of thousands of people caught up in this four-year conflict is that they feel cut off.
One woman told me they feel insecure because, she says, the government has blocked the roads. She explains people here fear further attacks.
Restrictions on the roads are also limiting the access of aid agencies. Medical, food and water supplies are struggling to get through.
And it is the multitude of rebel groups as well as government-backed militias which are contributing to the insecurity right across Darfur.
The UN's new envoy warned that morale amongst humanitarian staff was now fragile and said it could take only one major incident to trigger a humanitarian collapse - an issue Mr Holmes is expected to raise with Sudanese ministers when he meets them on Monday.
"I hope the fact that I'm new can enable me to make a bit of a new start with the Sudanese government here.
"That's what I'm trying to do. And if I can make some forward progress on that basis so much the better," Mr Holmes said.
Winning security guarantees from the Sudanese authorities is bound to be hard, not least because the international community is again talking tough, threatening sanctions on Sudan.