Syrian troops advanced in and around the northern city of Aleppo on Monday, in what appears to be an attempt to lay siege to opposition-held parts of the country’s largest city, activists said.

The troops faced rebels stretched thin by a two-front fight against government forces and Islamic militants encroaching on opposition-held areas. If rebels are driven out of Aleppo, it would be a near-fatal blow to an uprising that began in March 2011 as largely peaceful protests against President Bashar Assad’s rule, but later turned into a full-fledged civil war.

Aleppo is the last large urban area that Syrian rebels hold after losing territory to government forces over the past year, and it lies close to the border with Turkey, an important friendly supply route for rebels. Raqqa, further east, is held exclusively by Sunni extremists from the Islamic State group. […]