Syrian rebel forces have seized the army's infantry school in the town of Musalmiyeh, 16km north of the city of Aleppo, a senior military defector based in Turkey and rebel sources inside Syria said.

"This is of big strategic and symbolic importance.

“The school has ammunition depots and armoured formations and it protects the northern gate to Aleppo," Brigadier General Mustafa al-Sheikh told Reuters by phone from the town of Apayden on the border with Syria.

Earlier today, witnesses said Syrian forces had bombarded parts of Damascus with helicopter gun ships, driving rebels out of a northern district a week after opposition fighters launched a major assault on the capital.

Members of a Syrian army division under the command of President Bashar al-Assad's brother summarily executed several young men, a witness and opposition activists said.

Reports from Syria cannot be independently verified as the government has restricted foreign media.

In a further escalation of the conflict, fighting intensified around the intelligence headquarters in the biggest city Aleppo and in Deir al-Zor in the east.

Syrian forces regained control of one of two border crossings seized by rebels on the frontier with Iraq, Iraqi officials said.

However, rebels said they had captured a third border crossing with Turkey, Bab al-Salam north of Aleppo.

"Seizing the border crossings does not have strategic importance but it has a psychological impact because it demoralises Assad's force," a senior Syrian army defector in Turkey, Staff Brigadier Faiz Amr, told Reuters by phone.

"It's a show of progress for the revolutionaries, despite the superior firepower of Assad's troops."

The rebels said they lost 12 men and 40 were wounded during a 24-day siege they mounted to take the Bab al-Salam border post.

The rebel commanding officer in the area, Abu Omar, said they took the crossing point today without a fight.

"There were two armoured vehicles at the gate and they escaped. This is a safe zone for us and we don't expect the army to come and attack," said Mr Omar.

"Eighty percent of the area here is in rebel hands anyway," he added.

The bombardments in Damascus and Deir al-Zor were some of the fiercest yet and showed Mr Assad's determination to avenge a bomb on Wednesday that killed four members of his high command.

It was the gravest blow in a 16-month-old uprising that has turned into an armed revolt against four decades of Assad rule.

Rebels were driven from Mezzeh, the diplomatic district of Damascus, residents and opposition activists said, and more than 1,000 government troops and allied militiamen poured into the area, backed by armoured vehicles, tanks and bulldozers.

Three people were killed and 50 others, mostly civilians, were wounded in the early morning bombardment, a Mezzeh resident said.

The neighbourhood of Barzeh, one of three northern areas hit by helicopter fire, was overrun by troops from the Fourth Division, commanded by Mr Assad's younger brother, Maher al-Assad, 41.

"At least 20 Fourth Division tanks and hundreds of its members entered Barzeh this afternoon," opposition activist Abu Kais said by phone from the district.

"I saw troops go into the home of 26-year-old Issa al-Arab. They left him dead with two bullets in his head. 17-year-old Issa Wahbeh was pulled from a shelter and beaten and killed. Four other males in their 20s were killed this way,"

Mr Assad has not spoken in public since the bombing that killed four of his key allies.

Diplomats and opposition sources said government forces were focusing on strategic centres, with one Western diplomat comparing Mr Assad to a doctor "abandoning the patient's limbs to save the organs".

Mr Assad is still in Damascus and retains the loyalty of his armed forces, the Israeli military said, after questions had been raised about the Syrian leader's whereabouts.

Syrian state television quoted a media source denying that helicopters had fired on the capital.

"The situation in Damascus is normal, but the security forces are pursuing the remnants of the terrorists in some streets," it said.

Other opposition and rebel sources say the guerrilla fighters in the capital may lack the supply lines to remain there for long and may have to stage 'tactical withdrawals'.