Thousands of people have fled from a refugee camp in northern Lebanon, after a third day of fierce clashes between militants and Lebanese troops.

The Nahr al-Bared camp, home to 40,000 refugees, has been under siege since dawn on Sunday when fighting broke out, leaving residents without water, electricity and essential supplies. The Lebanese army has been bombarding positions in Nahr al-Bared with artillery, in its fight against the Fatah al-Islam movement. Twenty-seven civilians have been killed as well as 22 militants and 32 soldiers.

Christian Aid partner, Association Najdeh, which runs development projects in Nahr al-Bared, is trying to get water and food to those left in the camp, but estimates around 20,000 people may already have left for the nearby Bedawi camp and the city of Tripoli.

The United Nations Relief and Works Agency (UNRWA), has sent trucks of bread, milk and medicines to the besieged camp, but access is limited. The Palestinian Red Crescent estimates there are around 100 injured civilians in need of evacuation but medics have been unable to gain safe access to parts of the camp. The cramped conditions and lack of vehicular access in the camp makes it harder to get to those in need.

'We must move quickly,' said Leila El Ali, Najdeh's director, speaking about the urgent need for water and medical supplies for those left behind in the camp. 'The problems for those displaced will become more acute - there is limited access for non-governmental organisations to reach Nahr el-Bared and Bedawi...we don't know what is going to happen.'

Najdeh have made their kindergarten in the camp available as a shelter but yesterday a mortar struck very close to its vocational training centre in the camp, causing some damage. No one was injured.

The current internal clashes are the bloodiest since the end of Lebanon's lengthy civil war in 1990. Christian Aid and its partners are concerned about the safety of the Palestinian refugee civilian population and the potential ramifications for the wider Lebanese population. We emphasise the urgency of the call by Association Najdeh and other humanitarian organisations that both the Lebanese army and the militants allow safe passage for ambulances and humanitarian supplies into the camp.

'Among the civilian population, the poorest, the youngest and the oldest bear the brunt,' said Robin Greenwood, Christian Aid's head of Asia and the Middle East who has just returned from the country. 'People in the Palestinian refugee camps are particularly at risk because of their already vulnerable status, and lack of rights, which leaves them corralled in camps and therefore on the front line between militia and government forces. The Lebanese government's response to security threats must be proportionate.'

There are 391,000 registered Palestinian refugees living in Lebanon, who left their homes after the creation of the state of Israel in 1948. Approximately four million Palestinian refugees and their descendants live throughout the Middle East, in Jordan, Syria, Lebanon and the Occupied Palestinian Territories. They remain some of the most disenfranchised, marginalised and poor communities in the region.