Annan said in Geneva on Thursday he would not stop trying to convince Khartoum that UN troops were needed in Darfur to prevent a return to fighting.
Omar Hassan al-Bashir, the Sudanese president, said on Tuesday that UN troops were out of the question and suggested there was a "colonial" agenda behind demands that they be deployed.
"No one, and least of all the United Nations, is interested in imposing anything like a colonial rule on one of its member states," Annan said.
UN and African Union (AU) officials say the 7,000-strong AU peacekeeping force currently in Darfur needs urgent and effective support from the UN.
Jean-Marie Guehenno, the UN peacekeeping chief, Said Djinnit, the AU peace commissioner, and about 40 officials from both organisations have been studying the idea of replacing the AU mission with a larger, better-equipped UN force.
Cycle of violence
Guehenno said at the end of a two-week assessment mission to Darfur: "The risk of fragmentation, of a new cycle of violence, after the rainy season is quite real, very real."
Djinnit said the AU force needs "a more robust mandate, but also more robust support from the United Nations".
Annan said he would keep trying
to get Sudan to accept UN troops
The Security Council hopes to see the UN force take over from the AU by early 2007, but the peacekeepers would not go to Darfur without Khartoum's approval.
Alpha Oumar Konare, the African Union's chief executive, said after meeting the Sudanese president that the country was not closing the door entirely on the deployment of UN troops.
He said: "The government of Sudan is not rejecting the role of the UN, but they want to clarify what will be the nature of this force."
AU soldiers are trying to protect two million refugees in the western Sudanese region and monitor a faltering ceasefire between rebels, government forces and their Janjaweed militia allies.
While clashes between pro-government forces and rebels have largely ceased since the agreement, fighting among the rebel factions has increased and there have also been clashes between members of rival tribes.