More than 1,000 families were trapped on Saturday in several neighbourhoods of the central city of Homs and under bombardment by regime forces, the Syrian Observatory for Human Rights said.
"More than 1,000 families, including women and children," are trapped in the Khalidiyeh, Jourat al-Shiah, Qarabees, old Homs and Qusour areas. "They have no food and no medical equipment," the watchdog's Rami Abdel Rahman told AFP.
In a statement, the Observatory sent out an "urgent call" to UN Secretary General Ban Ki-moon "and all those with a sense of humanity to intervene immediately, in order to put a stop to the continuous shelling."
The Britain-based NGO also called for the "evacuation and protection of dozens of injured."
The humanitarian situation of those trapped in the neighbourhoods in question -- especially Khalidiyeh and Jourat al-Shiah, -- is "very bad," Abdel Rahman said.
"More than 100 people are injured, many of them badly, and the lack of medical equipment means some of them will die," he said, adding there was also a lack of medical staff.
Home to several bastions of the rebel Free Syrian Army, Homs has been under intermittent attack by regime forces since the Baba Amr district was pounded for a month earlier this year before being retaken, the Observatory said.
An escalation of violence over the past week has engulfed several areas of the country, including Homs. France's foreign ministry said on Friday it was deeply concerned at reports of "an imminent, large-scale operation" by regime forces in Homs.
More than 14,400 people, most of them civilians, have been killed in violence since the anti-regime uprising broke out in March last year, and a putative UN-backed truce has failed to bring a cessation of hostilities.