Violence flared yet again around Syria on Saturday, with the opposition reporting outright executions around Damascus and voicing fears about a climactic battle for the city of Aleppo as regime forces approached.

Free Syrian Army fighters told CNN that two large columns of government troops were heading toward Aleppo, the Middle East nation's most populous city. One is moving from Latakia on the Mediterranean coast and the other from Damascus.

Mohamed Said, a spokesman in Aleppo for the Syrian Revolution General Commission, said late Saturday that members of the opposition Free Syrian Army are trying to head off some of the approaching troops by attacking them in Idlib province.

Fighting in the northern Syrian city has raged for days, causing widespread destruction and casualties and forcing tens of thousands of civilians to flee. The addition of thousands of more troops to the mix, and even bloodier battles to follow, has fueled worries within the rebel ranks and around the world.

In remarks Friday to the U.N. General Assembly, Secretary-General Ban Ki-moon voiced concern the situation may get even worse, saying the "ancient and storied" city is now "the epicenter of a vicious battle between the Syrian government and those who wish to replace it."

Rebels in Aleppo exude gritty confidence and possess growing clout. They say they control significant parts of the city and are working to wrest control of the entire sprawling metropolis from the much larger, better equipped forces of Syrian President Bashar al-Assad.

The regime reported deaths, injuries and the capture of dozens of "terrorists" -- the term it commonly uses for opposition fighters -- in several neighborhoods of Aleppo on Saturday. Opposition activists, meanwhile, said al-Assad's forces had unleashed persistent and powerful bombs and shelling, while rebels have launched their own attacks in multiple neighborhoods.

That includes a brazen push Friday to seize a state-run broadcasting building in Aleppo. Rebels pushed into the radio and TV complex and took over parts of it, before eventually withdrawing because of snipers and military shelling, the opposition Local Coordination Committees of Syria said.

The regime reported "a large number of terrorists killed and injured during their attempt to storm" the building. Free Syrian Army commanders say the broadcasts can no longer emanate from the site, because of bombardment by Syrian aircraft. A rebel flag has been planted atop the building, they added.

Elsewhere in Aleppo, in Khan al-Assal neighborhood, the state-run Syrian Arab News Agency reported regime forces had caused the deaths and injuries of "a big number of terrorists" and confiscated weapons.

Rebel fighters, though, offered a different take. Mustafa Abdallah, a Free Syrian Army commander, claimed his group's fighters killed a few dozen Syrian soldiers in a two-hour gunbattle and captured eight others. At least one rebel fighter died, he said. Rebels eventually retreated as helicopters shelled the area and military reinforcements advanced, Abdallah said.

The Local Coordination Committees of Syria also noted "fierce clashes" in the Meridien neighborhood as warplanes fired on the area.

Syria's civil war is hardly confined to Aleppo, however.

At least 145 people were killed across the country Saturday, according to the LCC.

Some of the worst bloodshed was in Damascus, especially the Tamadon neighborhood. The LCC reported that 12 people there -- out of 53 killed throughout the city, including four others "executed" in the Qazzaz neighborhood -- were "summarily executed" after prayers by government forces.

Syrian state-run TV, by contrast, featured a banner Saturday night that read: "Our valiant armed forces cleanse Tadamon neighborhood in Damascus entirely from the remnants of mercenary terrorists."

Susan Ahmed, a spokeswoman for the Syrian Revolution, said Free Syrian Army forces pulled out of the neighborhood "tactically to protect civilians."

Even so, she added that there had been fighting, shelling and fatalities throughout the day and throughout Damascus, including in areas around parliament, the president's residence and the main building for al-Assad's cabinet.

"Very high and violent" explosions continued to pierce the nighttime air early Sunday, as did persistent gunfire, she said.

Elsewhere around Syria's capital, "armed terrorists" attacked a bus carrying 48 Iranian Shiite pilgrims and kidnapped them, Iranian state media said. Syrian state-run media also reported the bus abduction and said it is under investigation.