Paris Fears Aleppo 'Slaughter', London Warns of 'Humanitarian Disaster'

World powers voiced fears Friday over an imminent all-out assault by President Bashar Assad's forces on Syria's second city Aleppo and called for "maximum pressure" to prevent a new massacre.

France echoed U.S. concerns that Assad was preparing to carry out a slaughter of his own people, and Britain warned that the expected offensive could end in a humanitarian disaster.

Columns of tanks, helicopter gunships and warplanes have poured into Syria's commercial hub over the past two days, with troops firing on a string of rebel neighborhoods in Aleppo's southwest on Friday in the battle for control of the city.

Such an assault could be a potential watershed in the 16-month conflict that has already claimed more than 19,000 lives according to opposition activists.

"With the build-up of heavy weapons around Aleppo, Assad is preparing to carry out a fresh slaughter of his own people," French foreign ministry spokesman Bernard Valero told Agence France Presse, reiterating a call on Assad to end the violence and step down.

The US State Department voiced similar concerns on Thursday.

"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.

British Foreign Secretary William Hague said he was "deeply concerned" about the reported buildup of troops and tanks around Aleppo, and that the regime "has already begun a vicious assault on the city and its civilian population".

"This utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster," he said in a statement.

"It will add to the misery being endured by the Syrian people, and plunge the country further into catastrophic civil war.

"The Assad regime must call off this assault."

Italy called for "maximum pressure" on Assad to prevent further killings.

"We need everyone to raise the pressure to maximum on Assad to ward off a new massacre," Foreign Minister Giulio Terzi said in a statement, expressing fears about the "distressing" situation in Aleppo.

The U.N. human rights chief said she was also alarmed by reports of atrocities in Syria and warned that civilians were at "grave risk" in Aleppo.

"I have been receiving as yet unconfirmed reports of atrocities, including extra-judicial killings and shootings of civilians by snipers, that took place during the recent fighting in various suburbs of Damascus," U.N. High Commissioner for Human Rights Navi Pillay said in a statement.

"It goes without saying that the increasing use of heavy weapons, tanks, attack helicopters and -- reportedly -- even jet fighters in urban areas has already caused many civilian casualties and is putting many more at grave risk.

"All this, taken along with the reported build-up of forces in and around Aleppo, bodes ill for the people of that city," she said.

Attacks were also taking place in other towns including Homs and Deir Ezzor with "devastating consequences", she said, adding that a "discernible pattern has emerged" in the way government forces clear areas they claim are occupied by the opposition.