Last week I met Yasir (not his real name), a father of five, in a camp in Darfur. He told me his village was attacked by government troops and that his family were forced to flee from their homes with nothing. When I asked him how he knew that these were government troops, he responded: "Because they got out of government helicopters in government uniforms."

Yasir's story was one I heard time and time again when I visited the al-Salaam and Abu Shouk camps, home to around 100,000 people in makeshift huts, on the outskirts of El Fasher, the capital of North Darfur. People with their homes destroyed, villages emptied and relatives slaughtered. Most shocking was that, in many cases, it is their government which is responsible.

The world has a number of pressing concerns, but we cannot afford to close our eyes to four million people on humanitarian relief; over two million internally displaced and up to 300,000 killed. It is the biggest humanitarian crisis facing the world today. We have a profound moral duty to work round the clock not just to alleviate the suffering, but to stop the fighting.

The Darfur crisis is complex. There are tribal militias fighting each other. There is banditry on a massive scale. And there are rebel troops taking the opportunity to ransack villages. But at its heart, there is a simple dynamic: the Janjaweed militia, sponsored by the Sudanese government, is driving people out for reasons of ethnicity. This is ethnic cleansing - and we cannot remain silent in the face of this horror.

First, there needs to be an unequivocal ceasefire. Second, talks are needed to establish the framework for a settlement that will underpin lasting peace. And third, the international peacekeeping force needs to be strengthened in terms of numbers and the power it wields.

In El Fasher, I met the commander of the African Union peacekeeping force. He has just 5,000 poorly equipped troops to oversee an area the size of France, with over two million people in 173 camps. This force needs to be much larger, better equipped and, vitally, have the link to the UN without which it will not be able to do its job properly.

The Sudanese government is insisting that only African troops be allowed in Darfur, but they know that the AU will struggle to put together a force to do the job effectively. Internationalising the peacekeeping force would be an important step towards increasing its effectiveness.

It is far from certain whether Khartoum is willing to adhere to the terms of the recent agreement in Addis Ababa. I saw for myself how well practised Sudanese government officials are at offering slippery explanations for the violence their regime is perpetrating.

Shortly after that agreement was made, Sudanese forces attacked Birmaza, the town where subsequent ceasefire talks with rebels were supposed to take place. I challenged officials on this latest display of contempt for the peace process and was given four different responses by four different people.

Darfur presents a test case for the international community and its ability to handle humanitarian disasters. It is speaking with one voice in its condemnation. It was encouraging, for example, that China, which has oil interests in Sudan, urged the Sudanese to sign up to the Addis Ababa agreement.

If the Sudanese fail to comply, we should be ready to freeze their Swiss bank accounts, extend travel bans and make it clear to the generals and politicians that the International Criminal Court will pursue them vigorously for the crimes being committed in their name.

While visiting one camp, I saw a perfectly kept hut with beds made and a satchel hanging on the wall. It was a remarkable symbol of the resilience and spirit of the victims caught up in this terrible conflict. We owe it to them to show a similar resolve in bringing about its end.