BEIRUT: Syrian rebels shelled government-held districts of Aleppo Sunday and detonated a government building in the city as part of a surge against regime positions, killing and wounding dozens of people.

“The mortar fire targeted regime-held neighborhoods, including some in the Old City where the rebels are trying to advance,” the Britain-based Syrian Observatory for Human Rights said.

Aleppo is divided between government and opposition control, and both rebel bombardments and regime airstrikes have intensified recently.

The mortar fire came as “Islamist rebel brigades blew up a building housing the Chamber of Industry in the Old City, which was being used as a headquarters by government forces,” the Observatory said.

There were casualties among the troops inside the building, it added without giving a precise toll.

State television said “terrorist groups have blown up several buildings ... including the Chamber of Industry,” and reported six fatalities. Lebanese channel Al-Mayadeen reported 25 dead.

The bombing, which rocked the city, was part of a campaign by rebels to push back against regime attempts to take opposition-controlled parts of Syria’s former largest city.

The fight for Aleppo is particularly important now, with analysts saying they expect Assad’s forces will try wrest as much of the city as possible before elections.

Rebels dug a tunnel to reach the chamber in government-controlled territory, Aleppo activist Hassoun Abu Faisal said.

There are now three active fighting fronts in Aleppo, Al-Mayadeen reported, with some areas now changing hands on a daily basis.

Abu Faisal and other activists in Aleppo have said weeks of intensified clashes have also included the government increasing its shelling and bombing of rebel-held areas. Rebel groups have also cut off electricity supplies to the northern city.

Government aircraft dropped crude “barrel bombs” on several opposition-held districts in Aleppo, including Baidine, where six people were killed, the Observatory said, while a strike on the neighborhood of Shaar killed four.

Another 45 people were killed throughout Aleppo province in Saturday’s violence, including 12 rebel fighters killed in clashes with regime troops and paramilitaries.

In the province of Latakia, meanwhile, regime forces and their paramilitary allies seized a coastal village Sunday, ending the rebels’ weeks-long control over an outlet to the sea.

Hezbollah sources told The Daily Star that an amphibious landing was used in the attack on the village of Samra, as fighting continued in the hamlet, which rebel fighters captured last month as part of the “Anfal” campaign in northern Latakia.

On the border with Iraq, the Iraqi military targeted a convoy of militants from the Islamic State of Iraq and Greater Syria (ISIS), in a rare attack inside Syrian territory.

Iraqi army helicopters hit a jihadist convoy in eastern Syria, killing at least eight people, in a show of strength just days before the country’s first general election since 2010.

It was the first strike inside Syria claimed by Iraq since the three-year uprising against President Bashar Assad erupted in March 2011.

The conflict has spilled across the border, contributing to the most dramatic rise of violence in Iraq since the country’s 2006-08 sectarian war, with over 600 people killed this month. “The army struck eight tanker trucks in Wadi Suwab inside Syrian territory as they were trying to enter Iraqi territory to provide the Islamic State [ISIS] with fuel,” Interior Ministry spokesman Brig. Gen. Saad Maan said.

ISIS emerged in Iraq in the wake of the U.S.-led invasion in 2003, and later expanded into Syria during the uprising against Assad.

Maan said “there was no coordination with the Syrian regime” on the strike.

“Our responsibility now is to protect our border and to protect the border from the other side, because there is no protection from the other side.”

The targeted vehicles were apparently traveling to the western Iraqi province of Anbar, where ISIS has been battling Iraqi security forces.

Elsewhere, rebels seized a regime base located outside Damascus and lying on the highway to Iraq, the Observatory said.

Activists posted video footage purporting to show the spoils captured at the air defense facility, where fierce fighting that began after midnight Sunday killed at least 14 soldiers, the Observatory said.

In the south, rebels disrupted a gas line supplying power to Syria’s southern provinces and tried to consolidate gains in neighboring Deraa and Qunaitra provinces. The Observatory said rebels, including the Al-Qaeda affiliate Nusra Front, seized large areas of strategic hills in Qunaitra, bordering Israel.

In Deraa, some 90 rebel and regime forces were killed in two days of clashes over strategic sites near the village of Nawa, the Observatory said Saturday.

It said 45 opposition fighters and 43 regime forces had been killed in the fighting that began Thursday, which saw rebels seize the Tal al-Jabieh army post and seize weapons and ammunition.