ALEPPO, Syria: Three car bombs tore Wednesday into the heart of Syria's second city Aleppo, killing almost 50 people, mostly troops, as the regime launched an offensive against rebels near Damascus, a watchdog said.

Rebel fighters killed at least 15 soldiers, when they attacked military posts in the northwest of the country, triggering fierce clashes, said the Syrian Observatory for Human Rights.

And the bloodshed spilled across the Syrian border when several shells from the conflict crashed into the Turkish town of Akcakale, killing at least five people and wounding nine, witnesses said.

In Aleppo, two car bombs went off in quick succession around Saadallah al-Jabiri Square near a military officers' club, ripping off part of a hotel's facade and flattening a two-storey cafe, an AFP correspondent reported.

A man, whose family owns a coffee shop overlooking the square, described the sound of the blasts as "terrifying".

"I ran to my parents' room and found their faces covered in blood," said the man, who identified himself only as Omar. "Most of the people rescued from under the rubble of the hotel were soldiers."

A third bomb exploded soon afterwards at an entrance to the Old City in the nearby district of Bab Jnein, the Observatory and a military official said.

At least 48 people were killed and almost 100 wounded, the Britain-based Observatory said, citing medics. "Most of them were regime troops," it added.

An official in Aleppo put the toll at "37 dead and dozens injured."

"We heard two enormous explosions, as though the gates of hell were opening," Hassan, a 30-year-old employee of a nearby hotel, told AFP.

"I saw thick smoke, and I helped a woman on the pavement whose arms and legs were completely dislocated," said Hassan, who gave only one name.

The owner of a shop a block away from the officers' club said: "I pulled out from the rubble a child less than 10 years old who has lost a leg."

Rebels also attacked a political intelligence branch in Aleppo as well as an old vegetable market where a large number of troops were posted, said the Observatory, which relies on a network of activists on the ground.

Overnight, rebels also destroyed two tanks.

 'The decisive battle'

"The rebels are now attacking regime troops in the heart of the city," Observatory director Abdel Rahman told AFP.

"Before, the centre was spared from violence. Now, we can say that barring some districts, Aleppo is no longer a safe city.

"This is part of the decisive battle, and the regime can no longer claim to control the city," he added.

Aleppo, with a population of 1.7 million people, has been one of the focal points of the conflict since mid-July, when the army promised the "mother of all battles" to clear the city of rebels.

Since Thursday, the fighting has become more intense, spreading at the weekend into the centuries-old, UNESCO-listed souk in the histoc heart of the city and sparking a fire that damaged hundreds of shops.

Bombings have increasingly become part of the unrest ravaging Syria, which began in March 2011 as peaceful protests for reform but has since escalated into an armed insurgency, with more than 31,000 people killed, according to activists.

On July 18, rebels carried out a massive bombing on a complex in Damascus, killing four security chiefs, including President Bashar al-Assad's brother-in-law and the defence minister.

Regime forces have since pushed the rebels to the outskirts of the capital, but they have lost control of several border crossings and are battling to retake Aleppo.

Iin the northwestern province of Idlib, rebels killed at least 15 troops when they attacked and destroyed three army posts in the village of Bdama, near Jisr al-Shughur, said Abdel Rahman.

"Intense clashes broke out afterwards," he said, adding fighting also broke out elsewhere in the province, a stronghold of the rebel Free Syrian Army on the border with Turkey.

Nationwide, violence killed at least 90 people on Wednesday, including eight civilians who died when helicopters strafed the village of Sahn in Hama province, central Syria, the Observatory said.

The UN-Arab League envoy to Syria, Lakhdar Brahimi, is due back in the region this week to try to revive talks aimed at ending the bloodshed, officials said.

Jan Eliasson, deputy to the UN chief, said he did not know if Brahimi would be able to enter Syria, but hoped to persuade the Assad regime to "go in the direction of a reduction of violence."