Turkey has begun deploying rocket launchers and anti-aircraft guns along its border with Syria after last week's downing of a Turkish military plane.
Columns of military vehicles have been seen moving from military bases to the border, close to where the jet crashed.
The Phantom went down in the sea after being hit by a Syrian anti-aircraft fire. The two pilots are missing.
Meanwhile, three people have been hurt in explosions outside a court complex in central Damascus, Syrian TV reports.
A thick plume of smoke could be seen and state TV broadcast pictures of burning cars after what it called a "terrorist explosion" in the car park of the palace of justice.
There are also reports of clashes in three suburbs of Damascus, where activists say 22 people have been killed.
The scale of casualties cannot be verified independently as Syria severely restricts journalists' freedom of movement.
'Clear and present threat'
Turkey's decision to reinforce its border with Syria comes two days after Prime Minister Recep Tayyip Erdogan announced a change in terms of its military engagement.
He told parliament that Syria was a "clear and present threat" and any "military element" that approached the Turkish border from Syria would be treated as a threat and a military target.
Syria said the Turkish F-4 Phantom plane was brought down in Syrian waters by anti-aircraft fire with a maximum range of 2.5km.
But Ankara says it was hit in international waters by a heat-seeking or laser-guided missile.
Extra troops have been sent to the border area and Turkish TV has shown pictures of a small convoy of lorries carrying anti-aircraft guns into a military base near the border town of Yayladagi.
Other military vehicles have travelled to the border town of Reyhanli in Hatay province, reports say.
More than 33,000 refugees have fled Syria and have crossed the border into the province.
Turkish President Abdullah Gul is due to discuss the heightened tensions with Syria at a National Security Council meeting on Thursday.
Russia has backed a proposal for a national unity government to take charge of political change in Syria but it has refused to endorse the removal of President Assad from power.
According to Western diplomats, UN envoy Kofi Annan's unity plan proposes a cabinet including members of the opposition and government and bars anyone who would undermine its credibility.
The idea will be discussed at a Geneva conference on Saturday involving the UN Action Group on Syria.
President Assad's role is not spelled out in the Annan proposal but Western diplomats say he would not be part of any unity government.
Russian Foreign Minister Sergei Lavrov said on Thursday that "we do not and cannot support any intervention or solutions dictated from abroad". He added that President Assad's future had to be decided through a "Syrian dialogue by the Syrian people themselves".
President Assad has described Syria as being in a "real state of war" and the UN's deputy envoy to Syria said on Wednesday that the violence "had reached or even surpassed" the levels seen in April when Mr Annan's ceasefire plan was agreed.
The UN says at least 10,000 people have been killed since pro-democracy protests began in March 2011. In June, the Syrian government reported that 6,947 Syrians had died, including at least 3,211 civilians and 2,566 security forces personnel.
Source: BBC News