Turkey reinforced its border with Syria with additional troops and antiaircraft weapons Thursday, amid heightened regional tensions ahead of a conference of world powers in Geneva this weekend.

A bomb explosion in the heart of Damascus further underscored the sense of urgency building around the gathering, convened by U.N. special envoy Kofi Annan in an effort to salvage faltering international diplomatic efforts to stem the bloodshed in Syria.

Although it has raised hopes of a breakthrough, Annan’s plan has also been met with skepticism based on months of failure, during which the violence in Syria has steadily escalated. In addition to reiterating calls for a cease-fire, the proposal calls for U.N.-led negotiations between elements of the government and the opposition, leading to an interim unity government, a new constitution and eventual elections.

The plan would leave intact Syrian government institutions, including the military, while excluding from negotiations those “whose continued presence and participation would undermine the credibility of the transition” and jeopardize future stability. That presumably means President Bashar al-Assad.

It also envisions a new U.N. Security Council resolution and the possibility of a U.N. peacekeeping force.

The United States and Russia have agreed that the proposal forms the basis for further discussions Saturday in Geneva, although neither they, nor Syria’s government or opposition, have publicly voiced approval of specific new elements. The foreign ministers of Britain, France and China also are scheduled to attend the meeting, along with U.N. and Arab League officials who will gather with Syrian opposition leaders in a meeting Sunday.

Annan’s hope is that the plan will finesse a logjam in which the opposition has made Assad’s departure a precondition of negotiations, while Russia has refused to approve any initiative that guaranteed his ouster or that of his government.

Russian Foreign Minister Sergei Lavrov, who will meet with Secretary of State Hillary Rodham Clinton in St. Petersburg on Friday, said any political transition must be approved by the Syrian people, who he said must also decide Assad’s fate.

The dispatch of Turkish reinforcements came just days after Syria shot down a Turkish air force F-4 jet on Friday and NATO said it would honor its commitments to defend alliance member Turkey if necessary.

A senior NATO official emphasized that although Turkey has kept the alliance apprised of its actions, the new deployments “are measures they are taking themselves.”

The official, who spoke on the condition of anonymity about the sensitive issue, said Turkey, during a NATO meeting Monday in Brussels, had not asked for any specific alliance action. “What they wanted was to make sure that NATO is with them if it comes to defense of their territory. We have plans for the defense of Turkey. They exist; they have long existed,” the official said.

After the downing of the jet, Prime Minister Recep Tayyip Erdogan had said Turkey had changed its rules of engagement along its 550-mile southern border with Syria and would treat any Syrian military approach as a threat.

After a five-hour meeting Thursday, Turkey’s National Security Council issued a sternly worded warning that it “will act with determination to use all its rights within the international law against this hostile act.” State television broadcast footage of a column of military vehicles towing antiaircraft guns and multiple-rocket launchers arriving at a military outpost in the border town of Guvecci. It was one of several military convoys that reporters saw converging on the border.

The Turkish-Syrian tensions have compounded the increasingly complex situation inside Syria, where the once-peaceful uprising against Assad that began 15 months ago has evolved into a full-blown armed rebellion engulfing large swaths of the country.

Thursday’s midday blast in central Damascus took the violence to the symbolic heart of the city. Syria’s state-run news agency said three people were injured in what it called a “terrorist blast” in the parking lot of the Justice Palace, just outside the walls of the historic Old City and across the street from the entrance to the ancient Hamidiyeh souk.

The bombing came after an assault on a privately owned television station in a town 14 miles south of Damascus in which three journalists and four security guards were killed, suggesting that rebels are beginning to target civilian institutions associated with the regime. The rebel Free Syrian Army said, however, that the attack was staged by defecting soldiers, who clashed with security services guarding the TV station.

Colum Lynch, at the United Nations, contributed to this report.