(CBS/AP) BEIRUT - Activists and opposition groups say Syrian troops and rebels have clashed for a second day in the northern city of Aleppo.
They say the overnight clashes with heavy machine guns were some of the fiercest to date in the heart of Syria's northern commercial hub.
Aleppo has been largely shielded from the violence that has plagued other Syrian cities over the course of the uprising against President Bashar Assad, now in its 17th month.
Aleppo-based activist Mohammad Saeed said dozens of rebels from the Free Syrian Army were now in the city. He said fighting was mostly in the Salaheddine district in the city center.
The Local Coordination Committees activist network and the Britain-based Syrian Observatory for Human Rights said Friday's fighting forced many residents to flee to safer areas.
Also, a Free Syrian Army spokesman confirmed the capture of a strategic border crossing with Turkey on Friday, claiming the rebels had seized "Syria's gateway to Europe".
Muhannad Issa, a Vice Commander Idlib Martyrs Brigade - a Northern battalion of the Free Syrian Army, said from Hatay in Turkey on Friday that the seizure of the Bab al-Hawa crossing was a major victory for the rebels.
"It is Syria's gateway to Europe through Turkey and the FSA has the control of Bab al-Hawa right now. The helicopters and the Syrian army might be shelling but it is the FSA that has control of the strategic gate and from now on it will be of no use to the Syrian government."
A Turkish official based in Reyhanli, on the Turkish side of the border gate of Bab al-Hawa, confirmed that the rebels had taken control of the frontier crossing, but had no information on the latest situation over on the Syrian side.
Fighting has flared across Syria this week, as battles have ravaged Damascus neighborhoods, death tolls have skyrocketed, border crossings have fallen to rebel fighters and a rebel bomb attack killed top members of President Bashar Assad's regime.
The U.N. Security Council unanimously approved a resolution Friday renewing the 300-strong U.N. observer force in Syria for 30 days, leaving the door open for a possible extension if the government stops using heavy weapons and the escalating violence is reduced significantly.
A group of United Nations observers left their Damascus hotel on Saturday as part of a small group who have been cut from the 300-strong contingent.
On Thursday, Russia and China vetoed a resolution backed by key Western nations that would have threatened non-military sanctions on Assad's regime.
But Issa dismissed its importance on Friday.
"When we started going out in protests we always said we had no one but God with us. We know how to get our rights back," he said.
Assad's national security chief, General Hisham Ikhtiyar, died Friday from wounds sustained in the bombing Wednesday that killed three others, including the defence minister and Assad's brother-in-law.
All were key to the government's efforts to stamp out the insurgency.
"The Damascus blast has indeed shifted the balance of power in favour of the FSA," Issa said on Friday.
The government is pulling its most powerful troops from around the country to reinforce Damascus, which allows rebels to swoop in and take over key areas after the soldiers abandon their positions or leave them only lightly guarded
"The Syrian soldiers have been taking refuge in the tanks, the helicopters, the shelling and nothing else. They don't have control over anything on the ground apart from their immediate area," Issa added.
Syria's unrest began in March 2011 when protests calling for political change met a violent government crackdown.
Many in the opposition have since taken up arms as the conflict has morphed into a civil war that activists say has killed more than 17-thousand people.
The government says more than 4,000 security officers have been killed.