BEIRUT, Lebanon — Rebel fighters in Aleppo, Syria’s largest city, said Wednesday that government forces had opened a ground assault, forcing them to pull back from parts of the city because their ammunition was running low, as new disputes arose around the contentious issue of foreign military support for President Bashar al-Assad, and for the opposition.

In Aleppo, several rebel commanders said shelling and bomb attacks early Wednesday had reached new levels of intensity.

Residents reported receiving ominous cellphone text messages asking them to cooperate with the government. One text, signed by the Syrian Army, read: “Dear brothers, informing about terrorists means you are saving yourself and your family.”

Both the opposition and Syrian state television said the Syrian military had tried to reclaim the strategic neighborhood of Salaheddiin, where much of the fighting has been concentrated.