An estimated 40,000 people have fled a town in eastern Syria after three days of heavy fighting, according to the United Nations.

A World Food Programme team visited the Shaddadeh area in Syria's oil-producing east this week and said thousands had fled.

A violence monitoring group told Reuters that rebels had seized Shaddadeh, and that 30 rebels and 100 Syrian government troops had been killed.

Northeastern Syria was hit by four years of drought before the revolt against President Bashar al-Assad started nearly two years ago.

The drought resulted in high rates of malnutrition among children, WFP spokeswoman Elisabeth Byrs said.

"The fighting and displacement only aggravates the misery of these people," she said, adding the agency had sent extra rations to the area this week.

Taking Shaddadeh brings the rebels closer to the provincial capital Hasakah, 45km to the north in the surrounding Hasakah province.

The fresh displacement adds to an estimated 2.5m people already uprooted within Syria.

Many are living in squalid conditions in schools and other public buildings converted into shelters, according to the UN.

Elsewhere, activists say some 150 rebels and government troops have been killed in fierce fighting for control of the international airport in the northern city of Aleppo and a major military air base nearby.

The director of the UK-based Syrian Observatory for Human Rights says that death toll is from fighting on Wednesday and Thursday.