BEIRUT -- Syrian forces threatened yesterday 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 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 Assad. Iran said those captured when their bus was commandeered on Saturday were pilgrims visiting an important Shia shrine on the outskirts of Damascus.

The abductions threaten to suck Iran deeper into Syria's civil war and the wider political brinkmanship around the region. Iran claims it has no fighting forces aiding 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.

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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 Assad's power such as ministries and government offices already breached once by a bombing last month that killed four members of Assad's inner circle, including his brother-in-law and the defense minister.

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 Assad. Local activist Mohammed Saeed said government warplanes joined in the attack by strafing rebel positions.

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.

It's clear, however, that Assad's troops cannot simply surrender parts of Aleppo -- a city of 3 million -- to rebels without exhausting all military means. Rebels also have managed to keep open supply corridors from the Turkish border, about 25 miles away, which could also be targeted by any government ground offensive.

U.S. Secretary of State Hillary Rodham Clinton plans to visit Istanbul later this week for talks with Turkish officials over the worsening crisis in Syria, the State Department said yesterday. Turkey has taken in tens of thousands of Syrian refugees and hosts some high-profile defectors from Assad's regime.