Nobel Peace Prize winner Archbishop Desmond Tutu has called for sanctions to be imposed on Sudan unless it agrees to UN peacekeepers in Darfur.
Two million people have been made homeless in Darfur
"The world can't keep saying 'Never again'," he told the BBC.
Sudan refuses to let the weak, under-staffed African Union mission be expanded and given a tougher mandate.
It has denied charges that it backs Arab militias accused of genocide in Darfur and says the problems there are being exaggerated.
The violence in Darfur has escalated in recent weeks, despite the signing of a peace deal earlier this year.
On Monday, UN Secretary General Kofi Annan said the government had renewed aerial bombing and had sent thousands of troops to the region.
This has hampered the operation to feed up to 3m people there, most of whom have fled their homes to live in camps.
Archbishop Tutu's call comes ahead of a UN Security Council meeting on Darfur and Sunday's "International Day for Darfur", with calls on people around the world to take part in events to put pressure on their governments to do more to end the suffering in Darfur.
"We have a horrendous tragedy unfolding in Darfur," the South African archbishop told BBC Five Live radio.
He said the UN should give Sudan a deadline and say to the government:
"If you do not agree to a peacekeeping force, you will have to face sanctions."
He also suggested that a UN force could be sent to into Sudan, even without the government's permission, under the guise of seeking to arrest the officials accused of war crimes.
However, the Security Council resolution which backed the creation of a UN force said it could only be deployed with Sudan's approval.
Russia and China have previously blocked tough measures against Sudan by the Security Council - both have important trade links to the Sudanese government.
Archbishop Tutu also accused the international community of taking crises in Africa less seriously than in other parts of the world.
"The harsh truth is that some lives are slightly more important than others... If you are swarthy, of a darker hue, almost always you are going to end up at the bottom of the pile."