UN Secretary General Kofi Annan has stepped up pressure on the government of Sudan amid growing international concern about the crisis in Darfur.
African Union soldiers could pull out as soon as next month
He warned Sudanese leaders they could be held personally responsible for what happened in the region if international troops were not allowed in.
Mr Annan said conditions there were already desperate and many could die if peacekeepers were refused entry.
Earlier the UN refugee chief warned of an impending "catastrophe".
Antonio Guterres said escalating violence in Darfur was now threatening the entire region.
Three years of conflict have left millions displaced and thousands dead.
The UN says the situation shows no signs of improving, and aid agencies say it is getting worse.
The African Union has said it will pull out its 7,000-strong peacekeeping force next month, but Sudan has refused permission for a UN force to replace it.
Mr Annan said continuing to block UN intervention could leave Sudan's leaders open to be "held collectively and individually responsible for what happens to the population in Darfur".
He added that Darfur would be discussed at a UN Security Council meeting on Monday.
Despite a peace deal signed in May between the government and one of the rebel groups, the violence on the ground is increasing.
Attacks on aid workers occur almost daily - 12 have been killed in the last three months.
Speaking in Geneva, Antonio Guterres, the head of UNHCR, said it was a situation which could not continue.
"I think we are facing a terrible disaster. War is starting again, violations of human rights are massive, situations of rape - these have all kinds of devastating forms of impact in the lives of this population and make us feel more and more uncomfortable because we are not able to help them.
"We cannot even have access to them. This is unacceptable, this has to stop."
Aid agencies believe their work will be impossible without an international force in Darfur.
Privately many in the UN fear the escalating violence over the last few weeks is the build-up to a major attack by government forces, says the BBC's Imogen Foulkes in Geneva, where the UNHCR is based.
If that happens, it could trigger another flood of refugees from Darfur into neighbouring Chad, where the UN refugee agency is already caring for 200,000 people.
Those camps are stretched to breaking point, Mr Guterres said, and a new refugee crisis along the borders could bring instability to the entire region.