Syria's civil war has again spilled over into neighbouring Lebanon, with gun battles in the northern city of Tripoli between supporters and opponents of President Bashar Assad's regime that left four dead.

Nine Syrian judges and prosecutors also defected to the opposition. It was the latest setback for the regime, which appears increasingly embattled with rebels making gains in northern Syria and near Damascus, the capital.

Germany's Federal Intelligence Service chief Gerhard Schindler predicted the Assad regime would not survive.

"Signs are increasing that the regime in Damascus is in its final phase," he was quoted as telling the daily Frankfurter Allgemeine Sonntagszeitung.

In Geneva, the United Nation's Special Representative for Syria and the Arab League, Lakdhar Brahimi, met with Russian Deputy Foreign Minister Mikhail Bogdanov and US Deputy Secretary of State William Burns to discuss the crisis in Syria. They said in a joint statement that the situation in Syria was "bad and getting worse," adding that a political process to end the conflict was "still necessary and still possible."