DAMASCUS, Syria — Syrian government forces pushed into the ancient town of Palmyra, where Islamic State militants appeared on the verge of collapse Thursday, while in Iraq, a military spokesman announced the start of a long-awaited operation to recapture the IS-held northern city of Mosul. The extremist group has been losing ground in Syria and Iraq for months under a stepped-up campaign of U.S.-led and Russian airstrikes, as well as ground assaults by multiple forces in each country. The retaking of Palmyra — a UNESCO world heritage site whose fall to the militants last May sent shock waves through archaeological circles and beyond — would be a significant victory for the Syrian government.