At least 40 people have been killed and dozens more wounded as several explosions ripped through the centre of Aleppo.

Four blasts have struck a government-controlled district close to a military officers’ club in the northern Syrian city of Aleppo, killing at least 40 people and wounding more than 100, opposition activists say.

Later on Wednesday, a Syrian mortar bomb killed five Turkish nationals when it landed in southeastern Turkey. The deaths in the border town of Akcakale mark the first time Turks have died from mortar bombs landing on their side of the border.

In Aleppo, “a medical source said that at least 40 people were killed and 90 injured,” said the Syrian Observatory for Human Rights. “Most of them were regime troops.”

Meanwhile, official television channel Al-Ikhbariya said 31 people were killed and dozens wounded.

The attacks within minutes of each other struck the main Saadallah al-Jabiri Square near a military officers’ club and a hotel.

Syrian state television reported of “terrorist explosions” in the city.

Al Jazeera’s Rula Amin, reporting from Beirut in neighbouring Lebanon, said there was still no clear claim of responsibility for the attacks.

“Fighting between the government forces and the rebels continue, but no one is making any progress. The civilians are paying the price for it.”

‘Gates of hell’

“We heard two enormous explosions, as though the gates of hell were opening,” Hassan, a 30-year-old man who works in a nearby hotel, said.

“I saw thick smoke, and I helped a woman on the pavement whose arms and legs were completely dislocated,” said Hassan.

A shopowner whose store is located 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.”

All government buildings in the area were closed, he added.

The northern city of Aleppo, Syria’s commercial hub and largest city, has seen intensified fighting between regime forces and rebels trying to oust President Bashar Assad, especially after the fighters launched a new offensive last week.

Aleppo-based activist Mohammad Saeed said the explosions went off minutes apart at one of the city’s main squares. He said the blasts appear to have been caused by car bombs and were followed by clashes and heavy gunfire.

In a statement, the SOHR said the explosions went off following a clash between guards at the military club and gunmen, suggesting the attacks may have been suicide bombings.

Increase in car bombings

Suicide and car bombings targeting security agencies and soldiers have become common in Syria, particularly in the capital, Damascus, during the course of the 18-month-uprising against Assad.

But such bombings have been rare in Aleppo, which was spared the mayhem that struck other Syrian cities during the first year of the revolt.

Then, in February, two suicide car bombers hit security compounds in Aleppo’s industrial center, killing 28 people.

Nationwide, at least 104 people were killed on Tuesday, 57 civilians, 26 soldiers and 21 rebels, the Observatory said.

Among them were civilians hit by intense shelling from the army against rebel-held areas of Damascus.

Syria peace envoy 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.”

The uprising against Assad that erupted in March 2011 ago has gradually morphed into a bloody civil war.

The conflict has killed more than 30,000 people, activists say, and has devastated entire neighborhoods in Syria’s main cities, including Aleppo.