BEIRUT — Syrian forces threatened Sunday to mount a "decisive battle" for Aleppo, even as rebels clawed toward the city's ancient center under intense bombardment and strafing from warplanes. In the capital Damascus, militiamen appeared to step up guerrilla-like forays in central districts that were once firmly in the regime's hands.
The twin fronts reflected the rising stakes for both sides and a possible significant evolution in rebel strategies. Opposition forces appear to be shifting toward more hit-and-run strikes in Damascus and elsewhere to tie up President Bashar Assad's forces and blur the lines between rebel and government-held territory.
The biggest prize of the ambush brigades so far -- 48 abducted Iranians branded as spies by rebels -- was put on display in a video that carried a warning that all Iranians in Syria would be "captured or killed" because of Tehran's strong backing for Mr. Assad. Iran said those captured when their bus was commandeered on Saturday were pilgrims visiting an important Shiite shrine on the outskirts of Damascus.
The abductions threaten to suck Iran deeper into Syria's civil war and the wider political brinksmanship around the region. Iran claims it has no fighting forces aiding Mr. Assad, but it has sharply amplified its criticism of countries supporting the rebels such as neighboring Turkey and Gulf states led by Qatar and Saudi Arabia.
In further signs of the growing proxy nature of Syria's conflict, Iran was forced to reach out to Turkey and Qatar with appeals to help return the captives.
The daylight hijacking on the main airport road in Damascus also raises questions about the regime's ability to control key parts of the capital and protect the symbols of Mr. Assad's power such as ministries and government offices -- already breached once by a bombing last month that killed four members of Mr. Assad's inner circle including his brother-in-law and the defense minister.
The pro-government Al Watan newspaper said the Syrian army is bracing itself for a "decisive battle" to clear Aleppo, Syria's largest city, from rebels.
Government gunners pounded rebel-held areas to keep militiamen from expanding their hold on the city center, dominated by a medieval castle whose loss would be a deep symbolic blow for Mr. Assad, fighting to retain power after a 17-month-old uprising that, activists say, has claimed 19,000 lives.
Local activist Mohammad Saeed said government warplanes joined in the attack by strafing rebel positions.
"Fighter jets to us are now as common as birds in the sky," Mr. Saeed said.
Mr. Saeed and the British-based Syrian Observatory for Human Rights reported heavy shelling and clashes mainly in districts flanking Aleppo's historic center: Salaheddine to the southwest and al-Sukkari and Hananou to the northeast.
An amateur video posted on the Internet claimed to show shells crashing into a narrow street in Salaheddine, sending a shower of glass shards and chunks of rubble. The authenticity of the video could not be independently verified.
The government claimed it had regained full authority over Damascus after driving out rebels from central districts. But residents reported loud explosions and gunfire echoing from several areas of the capital overnight and early Sunday.
Khaled al-Shami, an activist in Damascus, dismissed as "nonsense" the official reports that rebels were pushed from Damascus. He said rebels are increasingly using a tactic of quick-hit attacks to frustrate security forces and keep the capital unstable.
"The Free Syrian Army does not seek to hold territory in Damascus but rather stage hit-and-run attacks that drain the regime. The rebels are present and strong there," he said.