BEIRUT – Syrian warplanes on Tuesday struck a strategic rebel-held town in the country's north in an attempt to reopen a key supply route, activists said, as a U.N.-proposed cease-fire meant to start this week appeared increasingly unlikely to take hold.

The U.N.-Arab League envoy to Syria has suggested that both sides in Syria's 19-month-old conflict lay down their arms during Eid al-Adha, a four-day Muslim holiday that begins Friday. However, neither Syrian President Bashar Assad nor rebels fighting to topple him have committed to a truce, and international envoy Lakhdar Brahimi has not said how such a truce would be monitored.

Syria's stalemated civil war, which has frequently spilled over Syria's borders and threatens to destabilize an already volatile region, featured prominently in the final pre-election debate Monday in the U.S. between President Barack Obama and his Republican challenger, Mitt Romney,