From my office in Dumfries to a sprawling camp in South Sudan:

the contrast couldn't be more stark. But when I took the call from Oxfam, I knew there was no time to lose.

Having previously helped co-ordinate its response to the Boxing Day tsunami, the famine in West Africa and the civil war in Darfur, the urgency of the crisis in the world's newest country was clear. More than one-third of South Sudan's people, some 3.7 million, are facing emergency and crisis levels of hunger and need immediate help.

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Oxfam is providing life-saving support to those forced to flee the fighting which started late last year after political divisions led to inter-ethnic bloodletting. It is one of the few aid organisations to stay in Juba after the fighting broke out.

Since then, we have seen an alarming increase in the number of people facing hunger and in the number of children suffering from malnutrition. To prevent catastrophic levels of hunger we need a massive and rapid global surge in aid. We cannot afford to wait or fail.

Four years ago I took the decision to return to Scotland and take up a "normal" job at the Crichton Carbon Centre in Dumfries. However, in January, Oxfam asked if I would return to support their response in South Sudan. Just three weeks later, having been generously given time off by my employer, I was standing in a hot and stuffy Juba airport.

A day later, I was running a complex, large and fast moving humanitarian programme in an insecure and risky environment, with the rainy season just round the corner. One of our key programmes operated from a sprawling and ramshackle camp on the outskirts of the capital Juba, known as "UN house". It was a hot and wind-swept camp of about 17,500 people who, even though they had jobs and homes to go to, had no intention of returning for fear of being targeted.

As well as suffering from hunger and susceptibility to disease, women and children were vulnerable to a lack of security and protection within the camp. Just going to the toilets at night was risky. With financial support from the UK Government, we installed solar lighting in the latrine area, provided women with solar lanterns and erected screens; simple measures that made a huge difference.

But the need is great. Health posts and hospitals have been looted and destroyed and hundreds of health workers have been forced to flee for their safety. Aid agencies and the people of South Sudan are facing a new challenge. The rainy season has begun and one of the most pressing tasks is to get seeds and tools to people so that they can start planting their crops. If this doesn't happen soon, the chances of a decent harvest in the months to come will be lost, with devastating consequences for people's ability to feed themselves and their families.

South Sudan faces a "now or never moment" to avoid catastrophic levels of hunger and suffering. If we fail to act, millions will pay the price. A recent ceasefire offers some hope for an end to the fighting but the task facing aid agencies remains enormous; in many areas, the rains have turned roads into rivers of mud.One of the main reasons I valued my time with Oxfam was the quality of people I worked with; I was not disappointed in South Sudan.

Oxfam has helped more than 180,000 people in the country and 63,000 who have fled to Uganda, providing food, access to clean water and sanitation, household items such as mosquito nets, blankets and stoves and charcoal for cooking.

Oxfam is calling on governments to respond generously and without delay. A new appeal will be launched by the UN next week. And Oxfam today launches a public appeal to help fund its £15 million work programme to deliver aid to people in dire need in the crisis hit country.

I am back behind a desk in Dumfries but I am very proud to have contributed to a programme that continues to provide life-saving support. I urge people across Scotland to do the same and support Oxfam's appeal. People can donate at or text DONATE to 70066 to give £5. SMS texts cost £5 plus standard network rates.