AZAZ, Syria: The Syrian army unleashed its forces on Aleppo Wednesday, ordering an armored column to advance on the country’s second-biggest city and pounding rebels there with artillery and attack helicopters, opposition activists said.

As hostilities intensified near the Turkish border, Ankara said it was closing its crossing posts, although the United Nations said refugees fleeing Syria would be allowed through.

In the Syrian town of Azaz, a few kilometers south of the Turkish border, rebels appeared in control after heavy clashes over the past month during which they succeeded in driving out government forces, leaving the place a rubble-strewn ghost town.

Syria’s ambassadors to the United Arab Emirates and Cyprus – a married couple – have deserted their posts, becoming the latest officials to abandon the Damascus regime, rebels said.

The 16-month revolt against President Bashar Assad has been transformed from an insurgency in remote provinces into a battle for control of the two main cities, Aleppo and the capital, Damascus, where fighting exploded last week.

Assad’s forces have launched massive counter-assaults in both cities. They appear to have beaten rebels back from neighborhoods in the capital and are turning toward Aleppo, a commercial hub in the north.

North of Aleppo, the town of Azaz has been almost completely destroyed by heavy fighting.

Burnt-out armored personnel carriers sit on the roads where rebels hit them with rocket-propelled grenades. Most residents fled during the latest fighting, which drove Assad’s forces out over the past month and ended in the rebels taking the Bab al-Salam border crossing with Turkey Sunday.

Fighting in and around Aleppo is expected to prompt an exodus across the Turkish border, where some Syrian refugees are already complaining about poor conditions and have clashed with riot police in disputes over food.

Further south, Syrian forces used artillery and fired rockets Wednesday on the northern Damascus suburb of Al-Tel in an attempt to seize it from rebels, forcing hundreds of families to flee the area, residents and opposition activists said.

“Military helicopters are flying now over the town. People were awakened by the sound of explosions and are running away,” Rafe Alam, one of the activists, said by phone from a hill overlooking Tel. “Electricity and telephones have been cut off.” Opposition sources also reported helicopters and machine guns were firing on the neighborhood of Hajar al-Aswad.

The slum lies on the southern outskirts of the capital and has been a haven for rebels sneaking into Damascus from the suburbs.

In the north, opposition activists said thousands of troops had withdrawn with their tanks and armored vehicles from Idlib province near the Turkish border and were heading toward Aleppo.

Rebels attacked the rear of the troops withdrawing from the north, activist Abdelrahman Bakran said from the area.

State-run Syrian television painted a more favorable picture, saying government troops were imposing security and stability in and around Aleppo.

Military experts believe an overstretched Syrian army is pulling back to concentrate on fighting insurgents in Aleppo and Damascus, important power centers for the government, while leaving outlying areas in the hands of rebels.

In Aleppo, helicopters were seen firing missiles throughout Tuesday, residents said. Rebels were battling government forces by the gates of the historic old city. Troops fired mortars and shells at rebels armed with rifles and machine guns.

“I heard at least 20 rockets fired, I think from helicopters, and also a lot of machine-gun fire,” a resident, who gave his name as Omar, said by telephone.

Residents said fixed-wing jets had also flown over the city, followed by loud noises, though there were contradictory reports as to whether they had opened fire. Video footage posted by activists appeared to show a warplane firing its guns.

The uprising has entered a decisive phase in the past 10 days with rebels emboldened in the capital after an explosion killed four members of Assad’s inner circle inside a security headquarters.

The blow wiped out much of the top echelon of his military command structure and shattered the reputation for invulnerability that his family has held since his father seized power in a coup in 1970.

Western powers have been calling for Assad to be removed from power for many months, and now say they believe his days are numbered. But they fear that he will fight to the end, raising the risk of sectarian warfare spreading across one of the world’s most volatile regions.

Former British Prime Minister Tony Blair said Wednesday that the West must do all it can to persuade Assad to step down but warns that the country would face acute divisions if Assad is ousted.

With a loosely linked group of rebel forces trying to overpower the regime, Blair said in an interview with the Associated Press that he feared unifying the country would be the toughest challenge in a post-Assad Syria.

“It’s very clear that in the end this regime will change,” said Blair, who serves as special envoy for the Quartet on the Middle East, a diplomatic peace initiative. “What we’ve got to do is both to hasten its end because the truth is, it’s lost the ability and the legitimacy to govern the country, but we’ve then got to work with everybody to construct the aftermath in the right way.”

Syria heightened the alarm Monday by confirming that it had chemical and biological weapons and saying it could use them against external threats, prompting warnings from Washington and Moscow against using the arsenal.

In its strongest language yet, Russia told Syria that it was unacceptable to threaten to use chemical weapons, the Russian Foreign Ministry said Wednesday.

In a meeting with Syria’s ambassador to Moscow, Deputy Foreign Minister Mikhail Bogdanov “laid out in an extremely clear form Russia’s position on the inadmissibility of any threats of the use of chemical weapons,” the ministry said.

But Russia also accused the United States of trying to “justify terrorism” after failing to condemn the bomb attack that killed Assad’s officials.

Foreign Minister Serge Lavrov, referring to comments made by U.S. State Department spokeswoman Victoria Nuland indicating the attacks were not surprising, said: “This is a direct justification of terrorism.”

Meanwhile, Israel said if Syrian-backed Hezbollah guerrillas used the situation to take control of the weapons, it would “act immediately and with utmost force.”

Elsewhere in the country, activists reported a series of killings by government troops and pro-Assad militia.

In Damascus, activists said they found the bodies of 11 men executed by government forces in the district of Qaboun.

The accounts, like others from activists, could not be confirmed. Syria restricts access by international journalists.