The African Union has delayed any decision on plans to deploy African peacekeepers to Darfur.
The UN had been unable to reach children behind rebel lines
Sudan rejected an AU proposal to send some 2,000 peacekeepers in the region saying it could amount to colonialism.
AU officials said at a meeting in Addis Ababa they were still working on the terms of the force's mandate.
The UN says more than one million people have been displaced by conflict in Darfur and many face the threat of hunger and disease.
Ahead of the AU talks, Sudan's Foreign Minister Mustafa Ismail warned that the "security of Darfur is the responsibility of Sudan alone".
The Sudanese ambassador in Addis Ababa, Osman al-Said, said Sudan had agreed to accept only 300 troops solely to protect ceasefire monitors.
The AU says it will go ahead with this force and the Rwandan army is hoping to despatch half this force to Sudan in the next few days.
But the BBC's Mark Doyle says the political stakes are rising.
He say the AU risks losing face if the Rwandans are delayed yet if they turn up Khartoum might perceive that its position had weakened.
Meanwhile the UN says two rebel groups in the Darfur region have agreed to allow the vaccination of up to 500,000 children against polio and measles.
UN agencies have been unable to reach them until now, because they live behind rebel lines.
The organisation said it had reached agreement with the Sudan Liberation Army and the Justice and Equality Movement for the vaccination programme.
The vaccinations will begin around 21 August in North Darfur.
Millions of children in government-controlled areas have already been inoculated.
The EU says there is "widespread, silent and slow, killing"
The UN says armed Arab militiamen, sometimes helped by uniformed soldiers are continuing to carry out attacks in Darfur.
Fred Eckhard, spokesman for UN Secretary General Kofi Annan, said: "The security situation in Darfur remains tenuous, with more violence directed at and displacing civilians in North and South Darfur."
A fact-finding report by the European Union on Monday said it had found evidence of widespread killing and little government protection of civilians.
However, it said it had found no evidence of genocide by Arab Janjaweed militiamen against African farmers.
Sudan has warned it will not be able to comply with a UN demand to disarm the Arab Janjaweed militia by the end of August or face international action.
Sudanese Vice-President Ali Osman Taha said 6,000 Sudanese police and government troops were currently in Darfur, and there were plans to expand the force to 12,000.
But he said logistical problems were hampering deployment, which meant that fully disarming the Janjaweed and other forces, by the end of August would not be possible.
The Arab League has given some support to Sudan, calling on the UN to give it more time to resolve the conflict.
Sudan's government has said it is ready to resume peace talks with two rebel groups in the Nigerian capital, Abuja, on 23 August. The AU says the mandate of their forces will be discussed then. However, rebels are saying they will not take part.
Sudan denies it supports the Janjaweed and has angrily rejected the threat of foreign intervention.