People have asked why this blog is anonymous. While we were in Sudan, nearly all aid workers would only speak anonymously, for fear of retaliation from the government. But now that we've been expelled from Darfur, what do we have to fear?

The Sudanese government is always careful to make sure it has a bargaining chip. Although our programmes have been shut down, we still have concerns about the safety of our staff. Our Sudanese colleagues remain in the country, vulnerable to intimidation and harassment. Some have been interrogated, others are scared they will be next. Many organisations still have a small number of international staff left in Khartoum. Some had their passports taken by the government and have been told they will not be allowed to leave Sudan until the "closure" has been completed to the government's satisfaction.

By this they mean that we have handed over our assets - our vehicles, offices, technical equipment - and agreed to their numerous demands. One of these demands is paying our Sudanese staff six months salary as a redundancy package. This has no basis in Sudanese law, which stipulates we have to pay one month.

The government says it has increased it as a punishment because we "violated our humanitarian mandate". We strongly disagree, but are left with little option. Pay up, or staff will not be able to leave Sudan. They will be taken to court and stuck in the country for months. They are effectively being held hostage. When I was in Darfur, I was called in for questioning by the government's security personnel - I know how intimidating it can be.

It's extortion that the mafia would be proud of.

Many millions of dollars worth of assets have been taken by the government - initial estimates suggest up to $60 million. The forced redundancy package alone will cost agencies millions. Our staff have worked incredibly hard for us for many years, and we want to make sure they get enough money to see them through this difficult time. But humanitarian needs in Darfur are greater than ever and funds are urgently needed for the people there - not to meet the government's blackmail.

Even those staff who still have their passports are vulnerable, as in Sudan the government controls everything with a mountain of bureaucracy. To leave the country we need exit visas, just as we need travel permits to move within Sudan and permits signed by numerous different government departments to transport all kinds of vital humanitarian assistance, from food to barrels of fuel to money to pay salaries.

This system of control that allows the government to threaten our staff with not being allowed to leave is one of the things that makes it so difficult for other agencies to fill the urgent gaps in humanitarian assistance that has been left by our expulsion. In theory, it should be relatively straightforward. Donors could give extra funds to the agencies who are still on the ground; they could employ our local staff who have lost their jobs and are in need of work; and then they could expand and take over our activities. Unfortunately, in Sudan things are rarely that simple.

Every project an aid agency does needs a signed agreement with various government departments. To move into a new town or camp needs another agreement, which can take months to process. The government has taken all our assets, but has not yet agreed to hand them to other agencies. To buy new assets requires even more signed pieces of paper. Importing equipment not available in Sudan takes months to get through customs. All local staff appointments have to be approved by the government, who sit on the interview panel and shortlist candidates. So far they have not agreed that staff from the expelled agencies can be re-employed. Some of our staff are almost certain to have problems.

Even if the agencies still in Sudan have the funds and technical expertise to take over our programmes, getting permission to do so is likely to take many months - if it is granted at all. Already some agencies who have offered to move into areas where we used to work have been barred from doing so by the government's National Security officials.

The people of Darfur cannot wait. The situation in some camps is already desperate. As I detailed in my last post, some camps like Kalma and Zam Zam are running out of food, water, medicine and other vital aid. The rainy season is approaching and outbreaks of disease are a real possibility. Already there are reports that 10 people have died from a fatal diarrhoea outbreak in Zam Zam. Humanitarian officials trying to assess the outbreak have been blocked from entering the camp by government security agents.

In an ideal world, the decision to expel us would be reversed and aid could be resumed. But the Sudanese government is as stubborn as it is crooked, and that looks highly unlikely. But at the very least, the international community must ensure that staff who remain in Sudan are not at risk and the agencies that remain must be allowed to respond to the urgent needs across Darfur.

For the moment - while my colleagues in Sudan remain in danger and are effectively kept as hostages, and the government continues to use humanitarian aid as a bargaining chip for its extortion - this blog will have to remain anonymous.

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