The United Nations' top emergency relief official has warned that the security situation in Sudan's Darfur region is becoming more difficult.
More than a million people have fled their homes
Jan Egeland, who has just visited Darfur, said relief supplies had been looted and humanitarian workers attacked by militia.
More than one million people have been forced from their homes and thousands more killed by Arab militia since 2003.
The government has denied accusations it is supporting the armed gangs.
Last week, Khartoum and UN Secretary General Kofi Annan issued a joint communique in which the Sudanese government committed itself to disarming the Janjaweed militia.
Khartoum 'not doing enough'
Mr Egeland told a news conference progress was being made in reaching the refugees.
However, he said, Darfur was becoming too dangerous for aid workers.
"We are now in this moment of truth, which will last for some weeks.
"My worst scenario is that the security will deteriorate, that we will step back at a moment we have to actually step up [emergency relief]."
Mr Egeland said aid workers were being attacked and emergency supplies stolen.
He said the government had to do "much more to disarm the infamous Janjaweed militia".
The BBC's Susannah Price at the UN says Security Council members are holding talks about an American resolution which would impose a travel and arms ban on the Janjaweed militia and give the council the option of doing the same to Sudan's government.
UN and Sudanese officials are due to meet in Khartoum on Thursday to discuss Darfur, while the Sudanese government and rebels will also hold talks in the Ethiopian capital, Addis Ababa, on finding a political settlement.