BEIRUT, Lebanon — Explosions and heavy fighting rocked Syria’s two largest cities on Saturday, witnesses and activists said, as the Syrian government and rebel fighters struggled to gain an advantage in the country’s bloody, 17-month-old conflict.

Also, Iran’s state news agency reported that unidentified “armed groups” had kidnapped 48 Iranians on the road to the Damascus airport after the Iranians visited a religious shrine.

The agency quoted an official from the Iranian Embassy in Damascus as saying it knew of the pilgrims’ whereabouts and was trying to get them released, Reuters reported. It would be at least the third time since the uprising began more than a year ago that a group of Iranians had been kidnapped, apparently by rebel forces angered by Iran’s support for Syria’s president, Bashar al-Assad.

The latest kidnapping report could not be verified, and it was unclear who might have been responsible. But if true, the kidnapping and the greater use of heavy weapons — tanks, helicopters and jets are involved daily now — suggest that Syria’s civil conflict is expanding and intensifying as new tactics, players and areas are drawn into the battle for control.