SYRIAN forces threatened to mount a "decisive battle" for Aleppo even as rebels clawed towards the city's ancient centre 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 towards more hit-and-run strikes in Damascus and elsewhere to tie up Syrian president Bashar Assad's forces and blur the lines between rebel and government-held territory.
The biggest prize of the ambush brigades so far was 48 abducted Iranians branded as spies by rebels. They were 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.
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 neighbouring 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 Assad's power such as ministries and government offices -- already breached by a bombing last month that killed four members of Assad's inner circle including his brother-in-law and the defence minister.
The pro-government Al Satan newspaper said the Syrian army was bracing itself for a "decisive battle" to clear Aleppo from rebels.
It gave no possible timetable and for more than a week, activists have claimed the government is gearing up for an all-out offensive on the northern commercial centre -- another critical battleground for Assad's regime to survive.
Government gunners pounded rebel-held areas to keep militiamen from expanding their hold on the city centre, dominated by a medieval castle whose loss would be a deep symbolic blow for Assad. 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," he said.
Mr Saeed and the British-based Syrian Observatory for Human Rights reported heavy shelling and clashes mainly in districts flanking Aleppo's historic centre.
Syria's official news agency SANA described the Aleppo rebels as "Gulf and Turkish militias" -- more evidence of the deep regional fissures between Assad and allies such as Iran on one side and nations that back the rebels on the other.
The government also claimed it had regained full authority over Damascus after driving out rebels from central districts including near Abbassiyyin Square, a major roundabout. But residents reported loud explosions and gunfire echoing from several areas of the capital overnight and early yesterday.
Khaled al-Shami, an activist in Damascus, dismissed as "nonsense" the official reports that rebels were pushed from Damascus. He said rebels were 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.
US secretary of state Hillary Clinton plans to visit Istanbul later this week for talks with Turkish officials over the worsening crisis in Syria. Turkey has taken in tens of thousands of Syrian refugees and hosts some high-profile defectors from Assad's regime.
Turkey's state-run agency said Syria's first man in space had fled to Turkey and joined opposition forces. The Anadolu agency said Mohammed Ahmed Faris crossed into Turkey after reaching Aleppo in his fourth attempt to defect. Faris, who was born in Aleppo in 1951, was part of a three-man crew of a Soviet space mission in 1987.