GENEVA (Reuters) - An estimated 40,000 people have fled a town in eastern Syria after three days of heavy fighting between government troops and rebels, the United Nations food agency said.

Rebels seized al-Shaddadeh in Syria's oil-producing east on Thursday after the clashes which killed 30 of their fighters and 100 Syrian government troops, a violence monitoring group told Reuters.

"A WFP (World Food Programme) team visited the area and estimated that around 40,000 people have fled al-Shaddadeh to al-Hasakah city (the regional capital)," the U.N. agency told journalists in Geneva on Friday.

Northeastern Syria was hit by four years of drought before the revolt against President Bashar al-Assad started nearly two years ago, resulting 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, 45 km (30 miles) to the north in the surrounding Hasakah province.

The fresh displacement adds to an estimated 2.5 million people already uprooted within Syria, many living in squalid conditions in schools and other public buildings converted into shelters, according to the United Nations.

(Reporting by Stephanie Nebehay; Editing by Andrew Heavens)