Syrian forces close in on Aleppo

Syrian government forces appear to be preparing for a large assault on the commercial capital of Aleppo, where anti-government activists say helicopter gunships have opened fire on civilians and that the army has massed troops on the city's borders.

The bombardment of rebel-held areas intensified in the early morning, with military aircraft overflying the city at low altitudes.

Syrian President Bashar al-Assad's forces reportedly renewed ground and aerial combat in and around Aleppo on Friday, extending efforts to crush rebels in Syria's second-largest city.

Western nations have warned of a potential massacre in Aleppo, the country's most populous city.

The rebels say they have destroyed a number of tanks, but their claim cannot be independently verified.

Reportedly the rebels are vastly outgunned and outmanned by forces loyal to Syrian President Bashar al-Assad.

Activists have reported violent clashes around the Salah al-Din and Hamdanieh quarters near the centre of Aleppo.

An emergency call has gone out to doctors to come to Salah al-Din and help if they can, our correspondent says.

On Friday, the Red Crescent suspended some of its operations in Aleppo because of the heavy fighting.

Rebels have been stockpiling ammunition and medical supplies in preparation for the expected assault.

The US state department said on Thursday that credible reports of tank columns moving on Aleppo along with air strikes by helicopters and fixed-wing aircraft represented a serious escalation of the government's efforts to crush the armed rebellion.

"This is the concern: that we will see a massacre in Aleppo and that's what the regime appears to be lining up for,"  spokeswoman Victoria Nuland said.

Nuland said that despite US concern over the violence, the administration did not contemplate providing military.

Reuters has reported that Turkey, Qatar and Saudi Arabia are providing intelligence and weapons supplies to Syrian rebels from inside Turkey, and that the United States is helping coordinate through "middlemen".

Nuland dismissed comparisons between Aleppo and the Libyan city of Benghazi, the cradle of the uprising against Muammar Gaddafi, where warnings of a threatened massacre by Libyan government forces in March 2011 prompted the UN Security Council to authorise military action to protect civilians.

The heavy fighting around Aleppo and the capital Damascus marks a new phase of the violence that has gripped Syria since protests broke out some 17 months ago. It comes after a bomb attack that killed four of Assad's closest lieutenants in Damascus last week that led some analysts to speculate that the government's grip was failing.

Meanwhile, in a setback to Assad's government, Ikhlas Badawi, a parliamentarian, became the latest to announce that she had "defected" to Turkey.

Samir Nashhar, a member of the opposition Syrian National Council, told the AFP news agency that Badawi arrived in Turkey on Thursday, and will now head to Qatar, "which has agreed to receive her".

In a press conference, Badawi blamed the "Assad regime" for failing the people and called on Syrian parliamentarians and the international community to prevent massacres.

Earlier, one of the most senior figures to defect from Assad's inner circle, Brigadier General Manaf Tlas, put himself forward on Thursday as someone who could help unify the opposition inside and outside Syria on a plan for a transfer of power.

Tlas, speaking in a newspaper interview in the Saudi city of Jeddah, also said he was looking for support from Saudi Arabia and other powers.