Tensions escalated Saturday as forces stepped up security in Aleppo, activists said, a day after car bombs killed at least 28 people in the first such attacks on Syria's second city in a year-old uprising.

Snipers were deployed and at least three armoured vehicles seen entering Sakhur, an Aleppo neighbourhood with simmering dissent against the regime of President Bashar al-Assad, one activist said.

Some residents who lost family members in the bombings were frustrated by the authorities' refusal to hand over their bodies for funerals, the activist, who only gave his name as Mohammed, told AFP on the phone from Aleppo.

"There is a heavy deployment of the security forces in areas of Aleppo that have seen a lot of demonstrations," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.

Abdel Rahman said the regime was trying to stir up tensions between pro- and anti-regime camps, especially in the districts of Sakhur, Marjeh, Ferdaws and Salahedin.

Aleppo, a commercial hub, has been largely spared the violence that rights groups say has killed more than 6,000 people in Syria since it broke out in mid-March.

Friday's twin car bombs targeting security posts in Aleppo killed at least 28 people and wounded 235 others, state media said, blaming "terrorists."

State television said a "suicide bomber in a car packed with explosives" set off a payload outside a police station, and another targeted an intelligence base.

The rebel Free Syrian Army accused the regime of launching the attacks in a bid to divert attention from its bloody nationwide crackdown on the protest movement.

"This criminal regime is ... carrying out bomb attacks in Aleppo to steer attention away from what it is doing in Homs, Zabadani and elsewhere," its spokesman, Colonel Maher Nouaimi, told AFP.

A report citing unnamed US officials said the attacks were likely to have been carried out by the Iraqi branch of Al-Qaeda, along with two other bombings in Damascus in December and January.

The McClatchy Newspapers chain said the incidents appeared to verify Assad's charges of Al-Qaeda involvement in the uprising against his rule.

The blasts came as tank-backed troops advanced on pockets of resistance in the besieged protest hub of Homs, and as heavy security deployments nationwide thwarted planned protests against regime ally Russia.

Regime forces have been waging a withering assault on Homs that has killed more than 450 people in the past week, rights groups say.

On Saturday, another four people were killed by shelling and heavy machinegun fire in Baba Amr, the main neighbourhood of resistance to Assad's regime, said the Observatory.

In addition to the victims of the attacks in Aleppo, the Observatory said another 44 people were killed across Syria on Friday -- 28 civilians, nine soldiers and seven deserters.

UN Secretary General Ban Ki-moon condemned the Aleppo bombings and called for the violence to "cease immediately from all sides."

Amid growing fears the unrelenting violence could spill over into neighbouring countries, pro- and anti-Syrian regime gunmen clashed across the border in Lebanon, a security official said.

One man died and three others were injured in an accidental munitions blast that followed Friday's clashes in the northern coastal city of Tripoli, the Lebanese official told AFP.

Eight people were wounded in shooting which continued sporadically on Saturday morning, even after the Lebanese army deployed to separate the two sides.

Sunni-majority Tripoli has in the past few years been the scene of intense clashes between Sunni supporters of the anti-Syrian opposition and Alawite Muslims loyal to a Hezbollah-led alliance backed by Iran and Syria.

Syria's President Assad hails from the Alawite community, an offshoot of Shiite Islam.

As the international community struggles to find a diplomatic response to the Assad regime's assault on protest cities, the opposition Syrian National Council was to meet Saturday in Doha, Qatar.

A Western diplomat in London said the 22-member Arab League was likely to launch a "Friends of Syria" coalition and appoint a special envoy to Syria at a meeting this weekend.