Medical charity Doctors of the World has pulled out of northern Mali following a recent outbreak of fighting by Tuareg rebels.
The organisation told the BBC it is temporarily suspending nutrition and health services, fearing for its workers' safety.
A BBC correspondent says dozens have been killed on both sides and the conflict appears to be escalating.
The rebels want an autonomous region of Azawad in the Sahara desert.
The Azawad National Liberation Movement (MNLA) took up arms last month following the return of many Tuareg fighters from Libya, where they had fought with Col Muammar Gaddafi's forces.
Doctors of the World, a worldwide network of volunteer medical staff, works with remote communities in the north of Mali - thousands of whom are now fleeing the fighting.
"The conflict makes the situation very insecure for our teams and the population," director general Pierre Verdeeren in Belgium told the BBC's Network Africa programme.
"The population is leaving so it is very difficult for us to reach them, so we decided to suspended temporarily the activities to deliver primary health care, medicine and sometimes food," he said.
Fleeing the fighting
More than 20,000 people have fled from Mali to neighbouring countries to escape the fighting, the UN refugee agency said on Tuesday.
Most of the 10,000 refugees in Niger are sleeping in the open with little access to shelter, clean water, food or medicine, a spokesman for the UN High Commissioner for Refugees Adrian Edwards says.
Another 9,000 have gone to Mauritania, and 3,000 to Burkina Faso - mostly northerners living in the south fearing reprisal attacks, the BBC's Martin Vogl in Bamako says.
The homes and businesses of some northerners in the south have been attacked.
He also says the major northern town of Kidal - in the Sahara Desert - is on high alert, with the MNLA threatening to launch an offensive.
The Tuareg are a nomadic community who mostly live in northern Mali, northern Niger and southern Algeria.
Mali's Tuaregs have long complained that they have been marginalised by the southern government and have staged several rebellions over the years.
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