Syrian troops and rebels poured into Aleppo on 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 2.7 million people, now looked likely to drag on for "several weeks."
As the rebels boasted successes on the ground, exiled opposition leaders bickered over strategy, with a prominent dissident announcing he had been tasked to form a government in exile, drawing criticism from the main opposition bloc.
"The army and the terrorist groups have both sent reinforcements for a decisive battle that should last several weeks," the security source told AFP.
The rebels sent in backup from nearby Turkey, after seizing a strategic checkpoint just outside Aleppo. That gave them a vital supply route between their bases over the border and the strategic prize of the country's commercial capital.
"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 security source said.
The Syrian Observatory for Human Rights said Tuesday's fighting in Aleppo 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 Bashar al-Assad's regime against "terrorists."
"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," Observatory chief Rami Abdel Rahman told AFP.
They later seized a third police station in Hanano, the Britain-based Observatory said.
Rebels also launched pre-dawn attacks with rocket-propelled grenades against a military court, an air force intelligence headquarters and a branch of Assad's Baath Party, according to the watchdog.
Meanwhile, activists posted a video on YouTube purportedly showing members of the rebel Free Syrian Army executing regime loyalists in Aleppo.
The victims were members of the Al-Berri tribe, which has has long supported regime forces in violent crackdowns on demonstrations in Aleppo, Abdel Rahman said.
"But under no law -- international law or Islamic law -- should anyone execute a prisoner. This is criminal. This is revenge."
The troubled UN observer mission in Syria, that had been deployed to monitor a putative ceasefire, said the army was using helicopters, tanks and artillery against the rebel fighters, who seized large swathes of the city in an offensive launched on July 20.
It appealed to both sides to protect civilians as the International Committee of the Red Cross reported that 200,000 people had fled the Aleppo area amid "continuous raging violence" over the weekend.
The office of the UN High Commissioner for Refugees said some 7,000 people had taken refuge in the city's university dormitories and more were camped out in 32 schools, each housing 250-350 people.
In total, the figures indicate between 15,000 and 18,200 people in these centres alone, said UNHCR spokeswoman Melissa Fleming.
On Monday, rebels seized the strategic Anadan checkpoint, some five kilometres (three miles) northwest of Aleppo, securing a direct route to the Turkish border.
Elsewhere in Syria, the watchdog reported renewed fighting in the eastern city of Deir Ezzor, where it said more than 300 people had been killed in the province of the same name over the past month, most of them civilians.
It said 70 percent of the city's residents had fled and estimated that 500,000 of the 1.6 million inhabitants of the province, which hosts Syria's main oil fields, had been displaced.
Veteran dissident Haytham al-Maleh told reporters he had been tasked with forming a government in exile based in Cairo, an announcement that the opposition Syrian National Council described as premature.
"I have been tasked with leading a transitional government," Maleh said, adding that he would begin consultations "with the opposition inside and outside" the country.
Maleh, a conservative Muslim, said he was named by a Syrian coalition of "independents with no political affiliation".
When Assad falls, "we don't want to find ourselves in a political or administrative vacuum," said the 81-year-old human rights activist who has twice served time in the Assad regime's jails.
But SNC chief Abdel Basset Sayda chief described Maleh's move as a "hasty decision."
"We wish it had not happened," he told AFP. "It actually weakens the opposition."
There have been other attempts by the Syrian opposition to prepare for a post-Assad future but none have achieved a consensus among the rival factions.
Arab and Western governments that sympathise with the uprising against the Assad regime have repeatedly expressed frustration at the failure of the opposition to present a united front.