BEIRUT, Lebanon — Syrian rebels shelled an airport near Aleppo on Thursday in what was described as one of the first known instances of insurgents using captured heavy weapons, as opposition activists warned that fighting for the city, the country’s main commercial center, would likely intensify.

A Syrian activist said President Bashar al-Assad’s army appeared to be preparing for an all-out assault.

“We have seen military reinforcements making their way to Aleppo,” said Abou Firas, an activist in Aleppo who used a satellite Internet connection because telephone and Internet service from the city was cut off. “We were worried about massacres but now we are issuing a warning about a war of extermination to be launched by the regime.”

The news about the government reinforcements could not be independently confirmed because of restrictions on reporters. It came after the battle for Aleppo intensified on Wednesday when United Nations observers there reported that Syrian jets had fired rockets into contested neighborhoods and that rebels had commandeered tanks and other heavy weapons.