The United States said Thursday it appeared that forces loyal to Syrian President Bashar Assad were “lining up” for a massacre in the city of Aleppo, but again ruled out military intervention in the conflict. The U.S. State Department said that credible reports of tank columns moving on Aleppo along with airstrikes by helicopters and fixed-wing aircraft represented a serious escalation of the government’s efforts to crush an armed rebellion. “This is the concern: that we will see a massacre in Aleppo and that’s what the regime appears to be lining up for,” State Department spokeswoman Victoria Nuland said. “Our hearts are with the people of Aleppo, and again this is another desperate attempt by a regime that is going down to maintain control, and we are greatly concerned about what they are capable of in Aleppo.” Nuland said that despite U.S. concern over the violence, the Obama administration did not contemplate ending its self-imposed ban on direct military help. “We do not believe that pouring more fuel on the fire is going to save lives,” Nuland said. “The route out of this is not more violence ... the route out of this is an end to the violence and a beginning to a true political transition process.” Fighting raged in Syria’s second city Thursday as troops and rebels prepared for a head-on confrontation and pro-regime media warned of a looming “mother of all battles.” A security source told AFP the army was preparing for an all-out assault as clashes also shook parts of Damascus and other areas, with at least 114 people reported killed Thursday – 61 civilians, 32 regime troops and 21 rebels. “The special forces were deployed Wednesday and Thursday on the edges of the city, and more troops have arrived to take part in a generalized counter-offensive Friday or Saturday,” the security source said of Aleppo. Rebels also brought in reinforcements, with the source estimating that between 1,500 and 2,000 opposition fighters had arrived from outside Syria’s most populous city to reinforce some 2,000 already fighting inside Aleppo. “They are mainly present in the southern and eastern suburbs of the city, mainly Salaheddin and nearby districts,” he said. The airport is currently cut off from the city, as four of the five roads leading to it are under rebel control, he added. Rebels also said a regime assault appeared imminent. “The army’s reinforcements have arrived in Aleppo,” Colonel Abdel Jabbar al-Oqaidi, a spokesman for the rebel Free Syrian Army (FSA), told AFP via Skype. “We expect a major offensive at any time, specifically on areas across the southern belt, from east to west.” Oqaidi added that some 100 tanks and a large number of military vehicles had arrived in Aleppo, the country’s commercial hub. The Britain-based Syrian Observatory for Human Rights reported that regime forces pounded the Salaheddin neighborhood in the south and Jazamati in the east. On July 20 the rebels launched an all-out assault to overrun Aleppo a move analysts say is aimed at establishing a bastion close to the rebel military headquarters in neighboring Turkey. The newspaper Al-Watan, which is close to the regime, led Thursday with the headline “Aleppo, the mother of all battles,” adding that “the army continues to chase terrorists in the outskirts of Damascus and the province.” Citing an Arab diplomatic source, it added: “Aleppo will be the last battle waged by the Syrian army to crush the terrorists and after that Syria will emerge from the crisis.” Meanwhile, U.N. chief peacekeeper Herve Ladsous said Thursday there was “no plan B” for Syria. “There is one political process for the time being, that is the six-point plan of the Joint Special Envoy Kofi Annan,” he told reporters in Damascus. “And as you know, and has been said time and again, there is no plan B. There is no alternative to that.” Annan’s plan called for an inclusive political process, a cease-fire, humanitarian aid, release of detainees, freedom of movement for journalists and peaceful demonstrations to be allowed. “Syrians killing Syrians is something that should not continue,” Ladsous added. Intermittent clashes were also reported in the Damascus area, with seven people killed there and 16 others, including five children, killed in shelling on Yalda village just south of the capital, the Observatory said. The Observatory also reported clashes in the Yarmouk Palestinian refugee camp, where a resident reached by phone confirmed the fighting. Troops also pounded several other districts in southern Damascus, and heavy clashes were under way in Al-Hajar al-Aswad neighborhood as regime helicopters strafed the area, activists and residents said.“Last night was quiet but people woke up to the sound of explosions and shelling from seven o’clock in the morning,” an activist calling himself Abu Qais al-Shami told AFP. After a week of heavy clashes, activists say regime forces have largely regained control of Damascus, with just a few pockets of resistance remaining. Meanwhile, Brigadier General Manaf Tlass, one of the most senior defectors from Assad’s rule, met Thursday with the foreign minister of Turkey, Syria’s powerful neighbor which has a stake in shaping any post-Assad leadership. Turkey’s Foreign Ministry said Tlass had arrived in Ankara and joined Foreign Minister Ahmet Davutoglu for “iftar.” Tlass appeared briefly with Davutoglu at an official guest house, but made no statement. Erdogan said Thursday he believed Assad was “on his way out” and that Syria was preparing “for a new era.” He said a transitional government should prepare for a new constitution as well as presidential and parliamentary elections. In Doha, members of the Syrian National Council met Thursday to come up with a possible transitional administration that could step in as a stop-gap government if rebel forces drive out Assad. Qatar is a leading backer of the Syrian rebels. The meeting is the most comprehensive bid to bring together Syrian opposition groups and show world leaders a credible alternative to Assad.