The UN High Commission for Refugees has warned the number of Syrians fleeing to neighbouring countries could reach more than double the current number of 300,000 by the end of the year.

"There may be up to 700,000 Syrian refugees in neigbouring countries by the end of the year," Panos Moumtzsis, the UNHCR's chief co-ordinator for Syrian refugees, told reporters in Geneva on Thursday.

"We are running out of time," he added.

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Faced with the soaring need for aid, humanitarian agencies upped their call for funds on Thursday to $487.9m to sustain their operations until the end of the year.

At present, only $141.5m in funding is available, just 29 per cent of the overall request, Moumtzsis said, stressing the urgency of the appeal in the face of an "overwhelming increase" in the number of refugees fleeing the conflict.

In March, the UNHCR had registered 41,500 Syrian refugees and said it expected the number to rise to 100,000 by the end of this year, but that figure was surpassed in July.

The approach of winter made Thursday's appeal even more important, Moumtzsis said, adding that winterised tents, clothing, blankets and heaters were needed to prepare for the "very harsh" winter months.

'Bloodiest day'

Meanwhile, Syrian troops have pummeled districts in east of Aleppo, a day after more than 300 people were killed, making it the bloodiest day of the 18-month revolt, according to a London-based rights group.

The Syrian Observatory  for Human Rights reported army shelling of the Maysar and Hanano districts in the northen commercial city, while the Local Co-ordination Committees said heavy artillery was being used to bombard Sakhur district, also in the east.

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The LCC, a network of opposition activists on the ground, also reported heavy artillery fire hitting Suleiman al-Halabi Street in the city centre.

It also said that before dawn, a car bomb exploded at an army checkpoint in the northwest province of Idlib, about 25km south of Aleppo city, on the highway to Damascus.

The Syrian Observatory said that 199 of Wednesday's dead were civilians.

"This is the highest toll in a single day since March 2011. And this is only counting those whose names have been documented. If we count the unidentified bodies, the figure will be much higher," Observatory director Rami Abdel Rahman told AFP news agency by telephone.

Elsewhere on Thursday, clashes broke out in the central province of Homs, the coastal province of Latakia and the eastern province of Deir Ezzor, where one rebel was reported killed, the Observatory said.

On Wednesday, four security guards were killed in two explosions near the Damascus general headquarters of Syria's army, state TV reported.

A total of 14 people were killed when twin bombs rocked the headquarters of the armed forces general staff in the heart of Damascus in one of the most spectacular attacks of the uprising, the Observatory said.

Source: Agencies