Mobile phone and Internet services have been cut in Aleppo, Syria's second city, where a crucial battle is taking place between rebels and the army, the Syrian Observatory for Human Rights said on Thursday.
"Mobile telecommunications services and Internet have been cut off in the city of Aleppo since last night," it said.
An activist, who asked not to be identified, confirmed that the Internet, landlines and some mobile services were down in the northern city, Syria's commercial hub.
"MTN is down while Syriatel is working. But Syriatel is working only for calls, not the 3G Internet service," he said, referring to the only mobile providers in the country. "The landlines are also down."
A Syrian security source in Damascus told AFP that such cuts are "generally the precursor to a major military offensive."
The rebel Free Syrian Army has said it controls "50 percent" of Aleppo, where the army is bombarding rebel-held areas but has yet to advance on the ground.
The conflict in Aleppo has raged since July 20, with both sides sending in reinforcements for what the security source has predicted will be a protracted battle.