Thousands of families in Syria have fled their homes in the past two weeks due to heavy fighting between government forces and rebels and many face food shortages, the United Nations said on Friday.

Food prices have tripled in parts of seven provinces where the livelihoods of farmers and livestock herders are at risk of collapse because the wheat harvest is being delayed by a shortage of diesel, needed for machinery, it said.

"The overall situation is characterised by severe insecurity and ongoing fighting which means that U.N. agencies do not have access to many areas," Jens Laerke, spokesman for the U.N. Office for the Coordination of Humanitarian Affairs (OCHA), told a news briefing in Geneva.

Many of the 200,000 residents of Douma, 15 km (10 miles) north of the capital, have fled to central Damascus, OCHA said.

Activists said on Wednesday that residents in the "ghost city" recovered mutilated corpses after a rampage by militiamen loyal to President Bashar al-Assad and army shelling.

About 30,000 people have fled the eastern city of Deir al-Zor and moved north towards Hassaka and Al-Raqqa, OCHA said. 

Civilians are also leaving Hama, Idlib and Al-Raqqa and heading towards Aleppo.

The Syrian Arab Red Crescent has previously estimated that at least 500,000 people are internally displaced in the country.

Syrian authorities and officials from the U.N. World Food Programme (WFP) carried out a joint assessment of food security in seven provinces.