Rebel fighters are reported to have captured large parts of a big military base in northern Syria, the latest in a string of losses by government forces. The attack on Base 111 at Sheikh Suleiman, about 25km (15 miles) west of the city of Aleppo, on Sunday, was said to have been led by Islamist militants.Video posted online showed them seizing military vehicles, including a tank.The assault came as UN envoy Lakhdar Brahimi said he had held "constructive” talks with US and Russian officials.They had "explored avenues to move forward a peaceful process and mobilise greater international action in favour of a political solution”, the Algerian diplomat added."All three parties re-affirmed their common assessment that the situation in Syria was bad and getting worse. They stressed that a political process to end the crisis in Syria was necessary and still possible.”They reportedly agreed that a solution would be based on the core elements of the final communique issued on 30 June after international talks in Geneva which called for a transitional government.On Monday, Russia’s foreign ministry called for the start of a national dialogue, but stressed that "fundamental decisions” had to be "made by Syrians themselves, without outside interference”.Rebel forces are said to have entered Base 111, whose headquarters are in Sheikh Suleiman, on Sunday afternoon after weeks of fighting.