UNITED NATIONS - August was the worst month for casualties so far in Syria's 18-month conflict, the United Nations said on Monday, warning that the worsening "grim spiral of violence" could threaten the country's neighbors.

UN Special Coordinator for the Middle East Peace Process Robert Serry did not give specific figures, but the world body says about 20,000 people have died during the conflict, including a record toll of 1,600 for the final week of August.

"The month of August registered the highest number of casualties thus far, and this toll is growing," Serry said.

More than 250,000 Syrians have fled to neighboring Turkey, Jordan, Lebanon and Iraq to escape the violence - with more than 100,000 of those leaving in August alone.

"Tragically for millions of Syrian civilians the violence and killing continue to mount as a result of a dangerous militarization of the conflict," Serry told the UN Security Council as he briefed them on the situation in the Middle East.

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