Turkey has begun deploying rocket launchers and anti-aircraft guns along its border with Syria after last week's shooting down of a Turkish plane.
Columns of military vehicles have been seen moving from military bases to the border, close to where the jet crashed,the BBC reports.
The F-4 Phantom jet went down in the sea after straying into Syrian airspace. The pilots are still missing.
Meanwhile, explosions have been reported in the centre of Damascus, outside the palace of justice.
Syrian state TV said there had been a "terrorist explosion" in the car park and witnesses spoke of a thick plume of smoke in the area.
Clashes in Damascus
There are also reports of clashes in the Damascus suburb of Douma, where opposition activists say four people have been killed.
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.
Extra troops have been sent to the 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 and other major powers are considering a proposal from UN envoy Kofi Annan for a national unity government to lead political change in Syria.
Moscow has agreed to back the plan which, according to Western diplomats, proposes a cabinet including members of the opposition and government, but no-one who would undermine its credibility.
The idea will be discussed on Saturday by the UN Action Group on Syria.