Turkey says its military rules of engagement have changed after Syria shot down a Turkish plane that reportedly strayed into its territory.
PM Recep Tayyip Erdogan told parliament that if Syrian troops approached Turkey's borders, they would be seen as a military threat.
Meanwhile Nato has expressed its condemnation of Syria's attack as well as strong support for Turkey.
Syria insists the F-4 Phantom jet was shot down inside Syrian airspace.
The plane crashed into the eastern Mediterranean and its two pilots are missing.
Meanwhile, fierce fighting has been reported between the Syrian army and rebel forces in the suburbs of the capital Damascus.
Witnesses say it is some of the most intense violence in the area since the uprising against President Bashar al-Assad began more than a year ago.
In other developments on Tuesday:
- The head of UN peacekeeping operations, Herve Ladsous, said the monitoring mission in Syria would remain suspended because of mounting violence.
- Russia said its foreign minister Sergei Lavrov would attend an international conference on Syria in Geneva on 30 June
Mr Erdogan spoke of Turkey's "rage" at the decision to shoot down the F-4 Phantom on 22 June and described Syria as a "clear and present threat".
"A short-term border violation can never be a pretext for an attack," he said. The Turkish jet was on a training flight, testing Turkey's radars in the eastern Mediterranean, he said.
The two accounts
He made it clear that Turkey was adopting a "common sense" attitude, although that "shouldn't be perceived as a weakness".
"Every military element approaching Turkey from the Syrian border and representing a security risk and danger will be assessed as a military threat and will be treated as a military target," he said.
Turkey requested a meeting of the alliance's ambassadors in Brussels after invoking Article 4 of Nato's founding treaty, which entitles any member state to ask for consultations if it believes its security is threatened.
'Disregard for international norms'
In a statement, the alliance's 28 members said the shooting down of the plane was "unacceptable" and they stood together with Turkey "in the spirit of strong solidarity".
Nato Secretary General Anders Fogh Rasmussen said: "It is another example of the Syrian authorities' disregard for international norms. Nato allies will remain seized of developments."
Earlier, in a letter to the UN Security Council, Turkey described the shooting down of its reconnaissance plane as a "hostile act" and "a serious threat to peace and security in the region".
Turkey has also accused its neighbour of firing on a search and rescue plane looking for the F-4 Phantom jet, although it was not brought down.
Relations between the two countries were already highly strained before the F-4 was shot down.
Mr Erdogan has been outspoken in his condemnation of Syrian President Bashar al-Assad, whose government he accuses of brutally putting down opposition protests.
In Syria itself, opposition activists on Tuesday reported fierce fighting near Republican Guard positions in suburbs of Damascus.
The Syrian Observatory for Human Rights told AFP news agency that fierce fighting had broken out in Qadsaya and al-Hama, around 8km (5 miles) from the centre of the city. The UK-based organisation also said security forces had entered the Barzeh area of Damascus.
It said 10 people had been killed by shelling in Qadsaya and some 58 people had died in violence across Syria - 24 soldiers, 30 civilians and four rebels.
Syrian state TV reported that dozens of "terrorists" had been killed in al-Hama and many others taken prisoner, including some non-Syrian Arab nationals.
The Observatory and the Free Syrian Army also said there had been reports of a military helicopter being shot down in Idlib, but gave no details.
The reports cannot be verified.
Fighting was also reported in the old city of Homs where the International Committee of the Red Cross (ICRC) last week tried unsuccessfully to arrange the evacuation of civilians. The ICRC said on Tuesday it was returning to the city for a fresh attempt.