Syrian activists say government troops fired from helicopter gunships on several neighborhoods of Syria's second-largest city of Aleppo on July 27.
Army forces have reportedly been massing since July 26 on the city outskirts for an expected major offensive against rebels.
Colonel Abdel Jabbar al-Okaidi, a spokesman for the rebel Free Syrian Army, told AFP via Skype, "The army's reinforcements have arrived in Aleppo." He added that the troops were backed by some 100 tanks.
"We expect a major offensive at any time, specifically on areas across the southern belt, from east to west," he said.
Some residents are reportedly fleeing Aleppo's southwest neighborhoods. Nationwide, nearly 30 people were reportedly killed in Syria on July 27.
UN High Commissioner for Human Rights Navi Pillay has expressed concern over "unconfirmed reports of extrajudicial killings and shootings of civilians during fighting" in the suburbs of Damascus by regime forces.
Pillay said "the increasing use of heavy weapons, tanks, attack helicopters, and reportedly even jet fighters in urban areas" were putting civilians at "grave risk.
Meanwhile, UN Secretary-General Ban Ki-moon said on July 27 that he is "deeply" concerned about threats by Syria to use chemical weapons if attacked by foreign forces.
Speaking at a joint news conference in London with British Foreign Secretary William Hague, he also demanded Syrian authorities "categorically state that they will not use chemical or other weapons of mass destruction under any circumstances."
Both Hague and Ban expressed deep concern about the escalating violence in Aleppo.
Hague called the Syrian government's assault on the city of Aleppo an "utterly unacceptable escalation" of the conflict.
"I am deeply concerned by reports that the Syrian government is amassing its troops and tanks around Aleppo, and has already begun a vicious assault on the city and its civilian
population," Hague said.
"This utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster," he added.
In other news, Poland said it has closed its embassy in Syria and evacuated its diplomats because of the deteriorating security situation in Damascus.
The embassy had been representing U.S. interests in the country.
Polish Foreign Minister Radek Sikorski said he took the decision out of concern for the security of the embassy's staff.
With reporting by AFP and AP