ALEPPO, Syria – Syrian rebels were bracing for the “mother of all battles" in Aleppo, with European powers fearing a “slaughter" was imminent and calling for maximum pressure to prevent it. As reinforcements poured into the city in what has been said could be a potential watershed in the 16-month conflict, the former UN observer mission chief General Robert Mood said President Bashar Al-Assad's fall was a matter of time. Waves of troops have been pouring into Aleppo, Syria's largest city and strategic commercial hub. “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," a source said. Fighting for control of the city is now in its ninth day. There was a chorus of international concern over Aleppo, with a number of powers expressing fears of yet another massacre. Noting the massing of forces, US State Department spokesman Victoria Nuland said “this is the concern that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for. Our hearts are with the people of Aleppo. And again, this is another desperate attempt by a regime that is going down to try to maintain control, and we are greatly concerned about what they are capable of in Aleppo." French foreign ministry spokesman Bernard Valero said France shared US concerns over the rapidly deteriorating situation, and called on Assad to end the violence and step down. “Our message is that Assad must go," he said. In Geneva, UN human rights chief Navi Pillay spoke of atrocities in the regime's ongoing battle to cleanse Damascus of rebel elements and said Aleppo was also at risk. British Foreign Secretary William Hague expressed fears that “this utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster." The International Committee of the Red Cross (ICRC) is evacuating some expatriate aid workers from Syria due to the worsening security situation, moving them temporarily to Beirut, a spokesman said on Friday. Such withdrawals are rarely announced by the independent agency, whose staff have crossed front lines throughout the 16-month-old conflict. The ICRC's move follows a partial evacuation by UN aid workers earlier this week. “An unstable and deteriorating situation in several parts of the country has led the ICRC to temporarily relocate some of its staff outside of Syria," ICRC spokesman Hicham Hassan said in Geneva. Some staff would be moved over the next few days but continue to support the ICRC delegation of 50 expatriates and nationals remaining in Damascus, he told Reuters. Poland said it has closed its embassy in Syria and evacuated its diplomats because of the deteriorating security situation in the capital. Foreign Minister Radek Sikorski said Friday he took the decision out of concern for the security of the embassy's staff. The Polish Embassy in Syria has been representing the US interests in the country. – Agencies