About 2.5 million people caught up in Syria's fighting need aid, the United Nations has said, double the number it estimated in June.

The increase was caused by a summer upsurge in violence particularly in Damascus and Aleppo, UN officials said during a meeting with major donors, including the US and Russia.

"The situation for the people of Syria people is appalling and getting worse every day," John Ging from the Office for the Coordination of Humanitarian Affairs said.

The UN also almost doubled its appeal for funds to purchase food, medicine, sanitation equipment and other supplies to $347m from a previous $180m.

A total of $103m funding had already come in, said OCHA.

After today's meeting, the European Commission said it would give a further €50m ($63m), on top of its existing €69m contribution.

The Syrian Observatory for Human Rights says more than 23,000 people have died in an uprising against Syrian President Bashar al-Assad that has lasted more than 17 months.

Around 200,000 Syrians have fled to neighbouring Turkey, Jordan and Iraq.