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 and burning cars were strewn over a car park used by lawyers and judges. State news agency SANA said three people were wounded by the bomb hidden in one of the cars.
The guerrilla attack in Damascus coincided with a Turkish military buildup on its border to the north and a growing sense of urgency in Western- and Arab-backed diplomatic efforts to forge a unity government and end 16 months of bloodshed.
Turkish military convoys moved slowly towards the Syrian frontier, reacting to Syria's shooting down of a Turkish warplane over the Mediterranean on Friday. A Turkish official said they were reinforcing air defenses.
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) away.
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 and headed for Kilis province, 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 front line of Western efforts to press Assad from power, hosts over 33,000 Syrian refugees on its southeastern border as well as units of the rebel Free Syria Army (FSA).
"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.
The FSA has stepped up pressure on Damascus in recent weeks, Thursday's attack marking a new stage in the campaign.
On Wednesday, rebels stormed a pro-Assad Syrian television channel and militants have targeted police and security personnel barracks. In April militants fired rocket-propelled grenades at the Central Bank building.
A senior opposition official said Syrian opposition groups would reject a political transition plan proposed by peace envoy Kofi Annan unless it explicitly required Assad to step down before a unity government is formed.
Diplomatic sources at the United Nations said Annan's proposal did not stipulate Assad's resignation although it does say the unity government could not include figures who jeopardize stability.
"The proposal is still murky to us but I can tell you that if it does not clearly state that Assad must step down, it will be unacceptable to us," said Samir Nashar, an executive member of the international Syrian National Council.
Annan's transition proposal is one of the main topics that Russia, the other four permanent U.N. Security Council members and key players in the Middle East will discuss at a meeting in Geneva on Saturday, according to United Nations diplomats.
Rebel fighters locked in the war to topple Assad said there was no part of the plan they could accept, and they had lost patience with U.N. envoy Annan's peace-making efforts.
"This is just a new labyrinth. It is new silliness for us to get lost in and haggle over who can participate and who can't," said Ahmed, a Free Syrian Army (FSA) fighter in Homs, epicenter of the revolt against four decades of Assad family rule in which the more than 10,000 people have been killed, by a U.N. count.
A member of the rebel group in Damascus suburb was also dismissive.
"I'll be direct. The FSA is doing its work, and it is not looking to the outside world. We don't want a transitional government unless it is the one formed by rebel military councils. The world is conspiring against the Syrian revolution," he said.
In April, Annan tried to implement a ceasefire to quell violence before embarking on peace talks. But the truce failed to take hold.
Diplomatic sources at the United Nations said the plan Annan will now pitch on Saturday aims to start the political process without waiting for a ceasefire. (Reporting by Erika Solomon in Beirut; Jonathon Burch and Jonathan Hemming in Ankara; writing by Ralph Boulton, editing by Janet McBride)