BEIRUT — The Syrian army pressed its offensive against rebels on Tuesday, bombarding the suburban Damascus city of Douma, while President Bashar al-Assad said he wished his forces had not shot down a Turkish warplane two weeks ago.

The downing of the Turkish F4 in disputed circumstances aggravated tensions between Damascus and Ankara, which responded with high-profile military moves to ward off Syrian helicopters from Turkey’s border zone where rebels and refugees are camped.

A Syrian general and 84 soldiers were the latest to defect and flee to Turkey on Monday. But analysts say army and government defections have so far made barely a dent in Assad’s armor.

There were few signs that diplomacy might stem Syria’s 16-month-old conflict, in which Syrian opposition leaders say more than 15,000 people have been killed. World powers at the weekend made a show of unity by pledging to back talks on a transitional government. But they failed to narrow differences between the West and Russia over Western demands that Assad must go.