BEIRUT – The Syrian military opened a second urban front Friday, attacking the rebel stronghold of Homs with the most intense artillery barrage in months and putting opposition fighters there and in Syria's largest city, Aleppo, increasingly on the defensive.

Syria's civil war has been locked in a bloody stalemate, and embattled President Bashar Assad could extend his hold on power if he retakes Aleppo and Homs. Amateur video from Homs, a symbol of resistance, showed black columns of smoke rising from the city, as loud explosions went off every few seconds.

While Assad stepped up attacks at home, tensions with neighboring Turkey flared again Friday, reviving fears that the 18-month-old conflict in Syria could ignite a regional conflagration.