A Syrian military offensive backed by heavy Russian air strikes threatened to cut critical rebel supply lines into the northern city of Aleppo, as peace talks in Switzerland appeared to be in jeopardy on Wednesday. The government attack north of Aleppo that began in recent days is its first major offensive there since Russian air strikes began on September 30. Rebels described the assault as the most intense yet. One commander said opposition-held areas of Syria's largest city were at risk of being encircled entirely by the government and allied militia, appealing to foreign states that back the rebels to send more weapons. Chances of achieving a ceasefire at talks in Geneva appear to be receding as the government, supported by Russian air power, advances against rebels, some of them US-backed. The refugee crisis and spread of the Islamic State of Iraq and the Levant (ISIL) through large areas of Syria, and from there to Iraq, had injected a new urgency to resolve the five-year-old Syria war. US Secretary of State John Kerry called on Moscow to stop the bombing during the peace process. "We are beginning the talks, we are at the table and we expect a ceasefire," he said after a meeting in Rome of countries opposed to ISIL. The area around Aleppo safeguards a rebel supply route from Turkey into opposition-held parts of the city and stands between government-held parts of western Aleppo and the Shia villages of Nubul and al-Zahraa, which are loyal to Damascus. "The supply routes were not cut but there is heavy bombardment of them by the jets," said a commander in the Levant Front rebel group who gave his name as Abu Yasine. "The Russian jets are trying to hit headquarters and cut supply routes." The Russian jets had been working "night and day" for three days, he added, and reiterated the rebels' long-held demand for anti-aircraft missiles to confront the assault. "If there is no support, the regime could besiege the city of Aleppo and cut the road to the north," said Abu Yasine, whose group is one of the rebel movements that have received military support from states opposed to President Bashar al-Assad, funneled via Turkey.