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 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 Shiite shrine on the outskirts of Damascus.