ALEPPO (AFP/Reuters) - Syrian troops and rebels poured into commercial capital Aleppo Tuesday as both sides battened down for the long haul after 40 police were killed on day four of a pivotal battle in the nearly 17-month conflict.A Damascus security source said the offensive which the army launched on Saturday to recapture rebel-held areas of the city of some 2.7m people now looked likely to drag on for “several weeks”.Buoyed by the rebels’ success in resisting the massive assault by troops backed by tanks, artillery and helicopter gunships, a leading dissident announced in Cairo that he had been tasked with forming a govt in exile.“The army and the terrorist groups have both sent reinforcements for a decisive battle that should last several weeks,” the Damascus security source said. The rebels sent in backup from Turkey, after they seized a strategic checkpoint just outside Aleppo, giving them a vital resupply route between their rear-bases over the border and the strategic prize of the country’s northern city. “The Syrian army is surrounding rebel districts, and is bombing them, but it is going to take its time before it launches its assault on each neighbourhood,” the source said.The Syrian Observatory for Human Rights said Tuesday’s fighting was the fiercest so far in a contest that state media had billed as the “mother of all battles” in the struggle to defend President Assad’s regime against the “terrorists”. “Hundreds of rebels attacked the police stations in Salhin and Bab al-Nayrab and at least 40 policemen were killed during the fighting.”Meanwhile, Turkish Prime Minister Tayyip Erdogan and US President Obama discussed how they could work together to speed up political transition in Syria during a telephone call, Erdogan’s office said. “In the talks, they took up the co-ordination of efforts to accelerate the process of political transition in Syria, including Assad leaving the administration and the meeting of Syrians legitimate demands,” a statement from Erdogan’s office said.