I was phoned by Médecins Sans Frontières's emergency desk on the weekend before Christmas. The fighting in South Sudan had broken out a few days before, and I was asked if I'd take in an emergency team to get some extra surgical capacity into the areas where the fighting was really intense. We left the next day.

We went first to Bentiu, the capital of Unity state. There had been fighting in the town the day before; the markets had been trashed and looted, and the doctors had left the local hospital. We found a ward full of about 45 quite severely wounded people, so we set about trying to help them medically. I started operating that evening. But the situation was deteriorating, and there were rumours of a major attack on the town. The next morning we were evacuated.

The town was extremely tense as we left. Lots of men with guns were walking around. Small columns of people with bundles were heading out across the main bridge. There was a sense of expectation that things were going to get worse.

Not long after, government forces went in and took over Bentiu. By then the population had disappeared to small villages and to the bush. Another MSF team went in five days later but again was pulled out because of disturbances. Our compound was trashed and looted and broken up, so it remained a very tense situation.

From Bentiu, we flew to Nasir. There had been fighting in the area all week and MSF's hospital was full of casualties. We worked for 36 hours with the local MSF team, with the surgeon and I doing some complex cases. Then we left for Lankien, where more casualties were coming in.

Lankien is a small, remote town of mud huts. When we got there the town was crowded; the population of 7,000 had more than doubled with people fleeing the fighting; and the hospital was full of casualties and distressed people.

MSF's hospital is usually focused on infectious diseases so the first thing we did was set up a casualty centre and an emergency operating room in a tent. We treated 130-140 people with gunshot wounds over the next three or four weeks.

The majority were young men of 16 or 17, some younger, who had been injured in the fighting to the north of us in Malakal, or to the south of us in Bor. They were brought to us two or three days after being injured, with major gunshot wounds and fractures, all of which were contaminated with dust and becoming infected. It took a lot of work to prevent these wounds developing blood poisoning and sepsis.

There was also a significant number of civilians, including children, who had been wounded in the fighting. One young boy of 11 had been shot in the spine and was paralysed from the waist down. I operated twice, and although I was able to remove the bullets and repair the area I very much doubt he will be able to walk again.

Late one evening I was asked to see a girl of about 12 who had been convulsing and had other medical problems. We worked hard, and I was thrilled to see her in the morning looking very much better and with a good prospect of a full recovery. But I was anguished to see that her carer was her nine-year-old brother. Her father had stayed in Malakal, and I don't know what had happened to her mother. I had to wonder what the future held for this little girl.

The hospital itself was under great pressure. A lot of our admissions were ordinary people who had made the three-day trek from either Bor or Malakal, walking through the heat of the day in temperatures in the mid to upper 30s. They had no food and very little water and some of them were simply collapsing from exhaustion. The number of outpatient consultations trebled in our clinics, which were overwhelmed.

The displaced people had very few possessions and it was clear that they were anxious to move on as soon as they had rested. Many had nothing more than the clothes they stood up in and were sleeping out under the stars. People were desperately short of water and food. Only four of the town's 12 water pumps were working, and our malnutrition facilities were very quickly filling up with malnourished children.

It's imperative that we keep up our work to provide essential medical care and emergency surgical care across the country – that we manage to keep our teams safe enough to carry on working with the wounded and the people who have been displaced from their homes. I don't know of any other organisation that could do what MSF is doing there right now.

South Sudan is the world's youngest country, not yet three years old and it's tearing itself apart. My thoughts are with the people of South Sudan in their hour of need; we need to support them through this.