DAMASCUS - The Syrian army moved reinforcements to Aleppo on Wednesday as the battle for the commercial capital intensified, with Washington saying rebel gains may result in a safe haven inside the country.

Helicopter gunships strafed neighbourhoods, causing deaths and injuries, the Syrian Observatory for Human Rights said, as regular troops battled rebel fighters through the night and into Wednesday morning.

The Britain-based watchdog reported clashes in the district of Bustan al-Qasr in the south of the city. Fighting was also reported in the central Al-Jamaliya neighbourhood, close to the local headquarters of the ruling Baath party.

A "large number" of troops have been moved from the northwestern province of Idlib to join the fighting in Aleppo, a rebel spokesman said via Skype.

Free Syrian Army Colonel Abdel Jabbar al-Oqaidi said he believed the reinforcements were being sent because of the intensity of clashes in Aleppo, where several districts were "liberated" on Monday.

"There are clashes right now in Aleppo, so fierce that many of their troops are running away, while dozens of others are defecting on the spot," Oqaidi said. "Their morale is very low."

Home to 2.5 million people, Aleppo is Syria's second-largest city. It had been largely spared major bloodshed since the uprising against President Bashar al-Assad's rule erupted in March 2011 but, for the past six days, heavy fighting has raged between troops and rebels.

In the central city of Homs -- Syria's third largest -- a rebel was killed by a sniper in the Al-Qarabis district, the Observatory said.

Further north, in Hama province, 16 people were killed in an army assault on the village of Sharia, while two children were killed in dawn shelling of the town of Karnaz.

In Idlib province, shelling killed four civilians in the town of Kfar Roma, the Observatory added.

The group put the updated toll from nationwide violence on Tuesday at 158 dead -- 114 civilians, 31 soldiers and 13 rebels.

The watchdog also reported another eight bodies -- some of them burnt -- were found in the Barzeh district of Damascus, scene of fierce clashes in recent days. It was unclear when the victims, some of whom had their hands tied behind their backs, were killed.

Activists and regime sources say government forces have reclaimed most of Damascus after a week of heavy fighting with rebels, who remain in the city but are planning a guerrilla strategy.

'Safe haven'

Despite the setbacks in the capital, US Secretary of State Hillary Clinton said rebel gains elsewhere could enable them to set up a safe haven inside Syria.

"We have to work closely with the opposition because more and more territory is being taken and it will eventually result in a safe haven inside Syria which will then provide a base for further actions by the opposition," she said.

"We do believe that it is not too late for the Assad regime to commence with planning for a transition to find a way that ends the violence."

As the fighting raged in northern Syria, Turkey said it was closing its border crossings until further notice.

"We have taken such a measure for our citizens for security reasons," a Turkish official said. "This is an open-ended measure and the reopening depends on the developments on the ground."

The move comes after rebels seized two border posts in clashes with the regime's loyalist troops.

Overnight, chief UN military adviser General Babacar Gaye arrived in Damascus to join peacekeeping chief Herve Ladsous.

"We have to assess the situation," Ladsous told reporters, adding that the mandate of the troubled UN observer mission in Syria would not be extended beyond a month "unless there are significant developments regarding the level of violence and the use of heavy weapons."

Around 300 unarmed observers were progressively deployed in Syria to monitor an April 12 truce that never took hold.

Assad's government suffered a new blow on Tuesday as the head of its mission in Cyprus, Lamia al-Hariri, defected to Qatar, Arab television networks reported.

Another defector, General Manaf Tlass, called on Syrians to unite and start building a post-Assad future, in his first public appearance since his July 6 defection.

Reading a prepared statement on Saudi-based Al-Arabiya television, Tlass called on Syrians to "unite ... to serve a Syria after Assad ... and do the impossible, to ensure the unity of Syria, and to be sure to start building a new Syria."