The Syrian army has stepped up an offensive on opposition Sunni Muslim strongholds in the central city of Homs, bringing in ground forces to try to secure passage for its forces through a major road junction, opposition sources said on Friday.The Syrian army has stepped up an offensive on opposition Sunni Muslim strongholds in the central city of Homs, bringing in ground forces to try to secure passage for its forces through a major road junction, opposition sources said on Friday.