Military intervention may be necessary if Sudan refuses to let aid workers reach the troubled province of Darfur, the United Nations chief has said.
Refugees say militias raid their villages after attacks by helicopter gunships
Secretary General Kofi Annan said the world could not stand by as it had during Rwanda's genocide 10 years ago.
Sudan's government says it wants more humanitarian aid, not military help.
The UN is investigating alleged "ethnic cleansing" by government-backed Arab militias. Some 100,000 people, mostly blacks, have fled Darfur for Chad.
Reports of atrocities in Darfur "leave me with a deep sense of foreboding," Mr Annan said in a speech to mark the 10th anniversary of the killings in Rwanda.
"It is vital that international humanitarian workers and human rights experts be given full access to the region, and to the victims, without further delay," Mr Annan said.
"If that is denied, the international community must be prepared to take swift and appropriate action. By action in such situations, I mean a continuum of steps which may include military action."
Leaders of Darfur's two rebel groups welcomed Mr Annan's speech.
Darfur rebel groups and the Sudanese government are holding talks in Chad, with international observers present.
The two delegations had a brief face-to-face meeting - their first - in the Chadian foreign ministry on Tuesday, and direct talks are expected to continue.
Reuters reports that the rebels from the Sudan Liberation Movement (SLM) and the Justice and Equality Movement (JEM) insisted on the presence of international mediators.
But the government says Darfur is a regional conflict and does not want to internationalise it.
Race against time
The fighting in Darfur, western Sudan, has been raging for more than a year.
Last week, UN emergency relief co-ordinator Jan Egeland accused the government in Khartoum of tolerating "ethnic cleansing" by Arab militias.
The rebels accuse the government of ignoring Darfur
The United Nations refugee agency, the UNHCR, says it is now a race against time to move the refugees further inland before the rainy season begins in a few weeks.
Speaking in Geneva, the agency's emergency coordinator for Chad, Yvan Sturm, said so far only around 22,000 refugees had been moved to safety and that aid workers faced major logistical problems moving the rest.
The UN is struggling with sand storms, no shade and no water.
But in just two months time the transport trucks will be immobilised, stuck on muddy, unsurfaced roads, when the rainy season begins.
The UNHCR wants to move 65,000 refugees hundreds of kilometres inland - away from the Sudanese border - before that happens.
Mr Egeland said the UN was getting daily reports of atrocities from Darfur.
He said it appeared to be an organised campaign of ethnic cleansing, with villages looted and burnt down and food and seed supplies destroyed in a "scorched earth" policy.
Mr Egeland said the international community should put pressure on Sudan to rein in the Arab militias.
But the Sudanese government has denied the allegations.