Akhir 12, 1432 / March 17, 2011, SPA -- U.N. Secretary-General Ban Ki-moon on Thursday expressed grave concern over the situation in Libya, where Moammar Qaddafi continues to regain territory taken by rebels over the past month. Ban said he is “is gravely concerned about the increasing military escalation by government forces, which include indications of an assault on the city of Benghazi.” The eastern city, Libya's second largest, is a stronghold of the rebel movement, but in recent days Qaddafi's forces have steadily moved closer. “A campaign to bombard such an urban center would massively place civilian lives at risk,” the U.N. chief said. Benghazi has a population of about 1.1 million people. “Those responsible for the continuous use of military forces against civilians will be held accountable,” he added. Meanwhile, the recently appointed U.N. special envoy to Libya, former Jordanian Foreign Minister Abdul Ila Khatib, departed Libya on Thursday after two days of discussions with senior Libyan officials. “The special envoy stressed the need for a firm and unambiguous commitment on the part of the Libyan government to cease hostilities immediately,” U.N. spokesman Martin Nesirky told reporters. “The special envoy's efforts continue, including contacts with representatives of Libyan political groups in Benghazi, as well as with the authorities in Tripoli.”