A group of exiled Syrian activists announced a new opposition alliance yesterday that aims to form a transitional government - a challenge to the Syrian National Council (SNC), a longer established group that they said had failed.

Meanwhile, rebels overran two police stations in Aleppo yesterday killing at least 40 policemen, a watchdog said, as fighting for control of Syria’s commercial capital raged into a fourth day.

'Hundreds of rebels attacked the police stations in Salhin and Bab Al Nayrab (neighbourhoods) and at least 40 policemen were killed during the fighting, which lasted for hours' said Rami Abdel Rahman, director of the Britain-based Syrian Observatory for Human Rights. The police chief was among those killed at the Salhin station in the south of the city, while three vehicles were destroyed, he added.

The launch of the 'Council for the Syrian Revolution' marks the latest effort by Syria's divided opposition to forge a political alternative to President Bashar Al Assad whose forces are trying to put down a 16-month armed uprising.

'The brothers have asked me to form a transitional government in Syria and to begin dialogue with the rest of the Syrian opposition,' Haitham Al Maleh, a former judge, told a Cairo news conference called to unveil the new body.

Maleh said the new alliance would act as an alternative to the SNC which he said 'had failed to help the Syrian revolution'. It would work to get more help to rebels, he said. Western and Arab states have for months been urging the Syrian opposition to unite. While the SNC has been an international voice for the opposition, activists on the ground have complained that the exiled leadership has little connection to what is happening in Syria.

Maleh, a long-standing dissident against the Assad family’s four decades in power, resigned from the SNC in March, saying he had given up trying to make the group more effective.

'We are not asking for military intervention, such as an invasion, but international protection, such as stopping Syrian planes,' he said. The Syrian military has stepped up a military campaign to drive rebels out of Aleppo, the country's biggest city. Outgunned rebel fighters are facing much heavier weapons including helicopter gunships. 'When Aleppo is freed, we will have the northern part of Syria and will ask the opposition to return home,' Maleh said.

The Council for the Syrian Revolution comprises 70 opposition figures and will be based in Cairo, with branches in Lebanon, Turkey, Jordan and Iraq.

'The movement includes members from the Syrian National Council yet its formation is meant to also tell the council that it is not the only body entitled to act on behalf of all the opposition,' said Ahmed Jalal Al Sayed, one of its members.