We were driving into Kandahar town after a medical assessment in Daman, a neighbouring village where there were numerous cluster bombs scattered in the fields, many of which were undetonated. Through the dust we spotted a big Ford pick-up with a westerner sitting in the back. He was wearing a T-shirt, and when we got closer we could see his machine gun, partly concealed.

He was one of the soldiers of the international coalition force, based near the airport in Kandahar. This turned out to be no isolated event. Time and again the Médecins Sans Frontières' team in Kandahar has observed military personnel from the international coalition force in civilian clothes with or without concealed guns, driving civilian cars. We have also met other special forces, probably British, in civilian clothes carrying guns, who claim they are on a "humanitarian mission" to assist NGOs in their work.

Despite wearing clearly marked white vests with our logo and name we have been directly asked by Afghans whether we are American soldiers. People suspect us of carrying hidden guns. We were repeatedly warned by Afghans not to go to specific places outside town since people might not be able to distinguish us from western soldiers. We have always known that female expatriates in Afghanistan can have problems with local traditions and suffer limitations on their work.

Now, male volunteers may be disadvantaged because they could be mistaken for soldiers. The Geneva Convention normally requires combatants to distinguish themselves from civilians and to wear uniforms or other visible and distinctive signs. Why is this not the case in Afghanistan? It is interesting that one of the arguments made by the US defence department to deny al-Qaida and Taliban detainees prisoner of war status is that they were not properly identified as combatants.

With the underlying tension between some Afghan military factions and the coalition forces, humanitarian aid workers are placed at significant risk because of this confusion. Kandahar was the ideological heartland of the Taliban. There was recent fighting at the hospital, in which the wounded remnants of the al-Qaida forces made a last stand.

There may be problems in the volatile rural areas outside Kandahar where MSF has recently begun working, and where small groups of al-Qaida and pro-Taliban forces may still be hiding. Our teams need to get out into the countryside to start mobile clinics for the large numbers of displaced people in Helmand province to the south-west of the town. We have been trying to expand our measles vaccination coverage.

And we have been carrying on our work in the infectious disease ward of the town hospital. But all of these activities depend heavily on a tradition of trust with the local people established before the latest war; before westerners came as killers as well as helpers of Afghans. What we have observed in Kandahar is part of a pattern by the international coalition, which blurs the distinction between military forces and independent aid workers.

We have already faced the problems of food drops alongside bombing. If armies engaged in the fighting are involved in delivering humanitarian assistance, it can be regarded by their opponents as an act of war. If humanitarian action is seen as partisan, aid and aid workers can be targeted. Humanitarian aid promotes a concern for humanity and dignity in times of violence. This relies on a respect for the impartiality of aid agencies, and their independence from the pursuit of military causes.

Every time soldiers involved in a conflict describe their actions as humanitarian, the vital concept of independent, neutral and impartial humanitarian action is eroded. This was clearly shown in Kosovo when Nato troops had a presence in refugee camps.

They were shelled by Yugoslav forces. International military personnel must wear something that allows them to be recognised in order to separate themselves properly from international aid workers. Coalition forces who wear civilian clothes misrepresent their role. This practice jeopardises the safety of humanitarian workers and endangers the humanitarian work, which is urgently needed and anticipated by the Afghan people.