The number of refugees fleeing Syria could reach 700,000 by the end of the year, the UN refugee agency said on Thursday, far surpassing its previous forecast of 185,000 reached in August.

About 294,000 Syrian refugees fleeing 18 months of conflict in their homeland have already crossed into four neighboring countries - Jordan, Iraq, Lebanon and Turkey - or await registration there, it said.

"This plan provides for up to 700,000," Panos Moumtzis, regional refugee coordinator for the U.N. refugee agency UNHCR, told a news briefing.

According to the UN's Refugee Agency, 2,000-3,000 Syrian refugees are added to the total each day.

The UNHCR report came after the Syrian Observatory for Human Rights said earlier in the day that more than 300 people were killed in Syria on Wednesday, in one of the bloodiest days in the 18-month uprising against President Bashar Assad.

World leaders meeting at the United Nations have expressed concern at the continuing violence in Syria but are deadlocked over their response to the conflict, which the Observatory says has claimed 30,000 lives since March 2011.

The British-based organization, which monitors violence in Syria through a network of activists, said in a report released on Thursday that 55 people were killed in rural areas around Damascus. They included at least 40 who appeared to have been shot in cold blood in the town of al-Dhiyabia, southeast of the capital.

Other activists have put the death toll in al-Dhiyabia as high as 107, blaming Mr Assad's security forces for what they said was a massacre. Video published by activists showed rows of bloodied corpses wrapped in blankets. The victims shown on camera appeared to be male, from 20-year-olds to elderly men.

The Observatory also said 14 people were killed in a rebel bomb attack on a military command centre in Damascus and in an ensuing prolonged gun battle between rebels and security forces.

Violence in Syria has deepened as the fight against Assad has become more militarised and the president has responded with increasing use of force - including regular air strikes and bombardments against rebel areas.

In the first nine months of the conflict, the United Nations human rights chief said around 5,000 people had been killed. UN officials have given up trying to monitor the violence but the Observatory's figures suggest five times as many people have been killed in the second nine-month period.

The Centre for Documentation of Violations in Syria, which is linked to the grassroots anti-Assad Local Coordination Committees, puts the overall death toll at 27,318.

The UNHCR report came a week after, Angelina Jolie traveled to Jordan where she met with Syrian refugees, who recounted the horrors that they have witnessed on their way to safety. In April, Jolie was promoted from serving as a UNHCR goodwill ambassador to a special envoy for the Syrian refugee crisis.