But rebel fighters are launching increasingly deadly attacks on regime targets, and several massive suicide attacks this year suggest al-Qaida or other extremists are joining the fray.
Assad told Iranian state TV Thursday that his country still supports him, and he warned that any intervention in Syria will cause a "domino" effect in the region. He has given similar warnings before, saying the entire Middle East will go up in flames if others meddle in his country.
Turkey, a former ally of Syria, has become one of the strongest critics of the Assad regime, and tensions between the two countries spiked following the downing of a Turkish military plane last week.
A small convoy of Turkish military trucks towing anti-aircraft guns entered a military outpost in the border village of Guvecci, which faces a Syrian military outpost, according to TRT television video.
Several anti-aircraft guns also have been deployed elsewhere along the border. Some trucks were seen carrying self-propelled multiple rocket launchers, the video showed.
Ties between Turkey and Syria have not been this low since the late 1990s, when the neighbors almost went to war. Tensions ran high in the 1980s and 1990s as Turkey developed military ties with Israel and Syria sheltered Turkish Kurd guerrilla leader Abdullah Ocalan.
Now, Turkey is hosting civilian opposition groups as well as hundreds of military defectors who are affiliated with the rebel Free Syrian Army.
Syria insists the Turkish military plane it shot down on June 22 had violated its airspace. Turkey says the jet had unintentionally strayed into Syria's airspace and was inside international airspace when it was brought down over the Mediterranean by Syria.
The search for two missing pilots continued in Syrian waters but hopes for their survival are dim, Turkish authorities said. The Turkish military said search teams have found some pieces of the plane as well as equipment belonging to the pilots.
Syria's state-run TV said the bombing occurred at 1 p.m. in the parking lot of the Palace of Justice, a compound that houses several courts. The site is near the capital's famous Hamidiyeh Market, an area crowded with families stocking up on food and other supplies for the weekend, which begins on Friday in Syria.
Witnesses reported hearing one blast, but state-run TV said there were two. The report also said a roadside bomb was found but did not explode.
An Associated Press reporter at the scene said some cars were charred and many had windshields blown out.
"I did not see any wounded people, but cars and nearby shops were damaged," said Fawaz Mishhim, who was in a nearby market when he heard the explosion.
The government blamed the attack on terrorists, the term it uses to describe rebels. Syria prevents journalists from working independently, making it difficult to verify accounts from either side in the conflict.
The country has been hit by a wave of explosions in recent months, killing dozens of people. Most targeted government security agencies.
Last month, an explosion targeted a military intelligence compound south of Damascus, killing 55 people. It was Syria's deadliest blast.
U.S. State Department spokeswoman Victoria Nuland condemned the latest bombing in Syria. She said that "the longer Assad continues on his current course, the longer he perpetrates violence against his own people, he creates the conditions for this kind of loss of control, including in the capital."
Also Thursday, Syria's state-run news agency, SANA, said terrorists assassinated a professor at the Petrochemical Engineering College at al-Baath University in the central province of Homs, along with five of her relatives.
Gunmen broke into her home and shot her, her parents and three nephews, SANA said.
World powers will meet Saturday in Geneva for talks on Syria, but few observers expect a major breakthrough. Syria has the protection or Russia, a veto-wielding member of the U.N. Security Council, and has so far been impervious to international pressure.
Russian Foreign Minister Sergey Lavrov said 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."
But U.S. Secretary of State Hillary Rodham Clinton said Thursday it was "very clear" that all the participants at the Geneva meeting including Russia are on board with a transition plan created by international envoy Kofi Annan.
Clinton told reporters that the invitations to Saturday's meeting in Geneva made clear that representatives "were coming on the basis of [Annan's] transition plan."
Lavrov said it was "obvious that a transitional period is needed to overcome the Syrian crisis," but insisted any plans for the future rest on Syria and that the major powers in Geneva must focus on convincing the opposition groups to soften their demands.
Diplomatic hopes have rested on Russia to agree to a plan that would end the Assad family dynasty, which has ruled Syria for more than four decades. Russia is Syria's most important ally, protector and supplier of arms.
There are few options besides keeping up diplomatic pressure, as an international military intervention is all but ruled out in the near future. Few countries are willing to get deeply involved in such an explosive conflict, and Russia and China have pledged to veto any international attempt to intervene.