BEIRUT - Rebel forces attacked Syria's main court in central Damascus on Thursday, state television said, while Turkey deployed troops and anti-aircraft rocket launchers to the Syrian border, building pressure on President Bashar al-Assad.
There was a loud explosion and a column of black smoke rose over Damascus, an Assad stronghold that until the last few days had seemed largely beyond the reach of rebels. State television described it as a "terrorist" blast.
Dozens of wrecked cars were strewn over a car park used by lawyers and judges. The Syrian flag few at half mast on the building, but there was no word on casualties in the attack on a potent symbol of Assad's authority.
Tension grew on Syria's northern borders.
Turkish military convoys moved towards the Syrian frontier, reacting to Syria's shooting down of a Turkish warplane over the Mediterranean on Friday.
The build-up of defences coincided with a general escalation of violence in Syria itself and a growing sense of urgency in Western- and Arab-backed diplomatic efforts to forge a unity government and end 16 months of bloodshed.
Prime Minister Tayyip Erdogan, speaking after Friday's incident, ordered his troops to treat any Syrian military element approaching the border as a military target.
He gave no details, but this could cover Syrian forces pursuing rebels towards the border, or patrolling helicopters or warplanes. Syria said at the weekend that it had killed several "terrorists" infiltrating from Turkey.
A first substantial convoy of about 30 military vehicles, including trucks loaded with anti-aircraft missile batteries dispatched from Turkey's coastal town of Iskenderun, was moving slowly towards the Syrian border 50 km (30 miles).
A Reuters reporter near the town of Antakya saw the convoy moving out of the hills and through small towns on a narrow highway escorted by police.
Early on Thursday, another convoy left a base at Gaziantep near the Syrian border and headed for Kilis province, which is the site of a large camp for Syrian refugees. Video from the DHA agency showed the convoy, of about 12 trucks and transporters, filing through the gates of the base past the hanging Turkish red flag with white crescent moon and star.
David Hartwell, Middle East analyst at IHS Jane's called the Turkish action a 'pragmatic, rational response' after the shooting down of the Turkish aircraft, that Syria insists was flying low and fast in Syrian air space. "Damascus has been warned once. I doubt there will be a second warning."
Turkey, in the forefront of Western efforts to press Assad from power, hosts over 33,000 Syrian refugees on its south-eastern border as well as units of the rebel Free Syria Army (FSA) that is fighting to overthrow Assad.
"I can confirm there are troops being deployed along the border in Hatay province. Turkey is taking precautions after its jet was shot down," a Turkish official said on condition of anonymity.
He said he did not know how many troops or vehicles were being moved but they were being stationed in the Yayladagi, Altinozu and Reyhanli border areas. He said anti-aircraft guns were being stationed along the border.
At a fortified Turkish border outpost near Reyhanli, a low building with red tile roof perched on a hilltop, troops stood on an observation platform looking across into Syria; but there was no sign of any unusual activity.
Members of the FSA, talking close to the border, told Reuters they did not believe the Turkish deployments were on a large scale or aimed at any crossborder intervention.
"The Turks know that any large scale military action would need international support," a senior FSA commander, who spoke on condition of anonymity, said.
Turkey has in the past spoken of opening a humanitarian corridor on Syrian soil, if the refugee flow grew unmanageable or if the violence and killing became intolerable.
Wary of igniting a regional sectarian conflagration, it has always insisted this would be possible only with United Nations backing. Western- and Arab-backed efforts to forge a joint diplomatic approach with Russia have so far failed.