People stand among wrecked cars at the site of an explosion outside Syria's highest court in central Damascus Thursday. — ReutersDAMASCUS – A strong explosion rocked the Syrian capital Thursday near a busy market and the country's highest court, while Turkey deployed troops and anti-aircraft rocket launchers to the Syrian border, building pressure on President Bashar Al-Assad. The blast came as tensions threatened to spread across the region. Major world powers will meet Saturday in Geneva for talks on Syria, but few observers expect a major breakthrough. 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 opposition fighters. State television described it as a “terrorist" blast. Dozens of wrecked cars were strewn over a car park used by lawyers and judges. A police source said magnetic bombs exploded in two judges' cars in the open-air car park, but a third did not explode. He said three people were hurt, without identifying them, and that 18 cars were damaged Turkish military convoys moved toward the Syrian frontier, reacting to Syria's shooting down of a Turkish warplane over the Mediterranean Friday. The build-up of defenses 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. 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 toward the Syrian border. Early 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. “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. Russian Foreign Minister Sergey Lavrov said Thursday that Moscow will not endorse a call for Assad to give up power. “We are not supporting and will not support any external meddling," he said. “External players must not dictate ... to Syrians, but, first of all, must commit to influencing all the sides in Syria to stop the violence." 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 UN 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 of the international Syrian National Council. — Agencies