The head of Nato said on Tuesday that the 28-member military alliance had plans in place to defend Turkey against attack from Syria if necessary, but that he hoped the two countries would find a way to stop tensions escalating.

Nato ambassadors threw their support behind Turkey in an emergency meeting last week after Syrian shells struck a border town in Turkey killing five civilians.

The two neighbours have repeatedly exchanged fire since then, the most serious outbreak of cross-border violence since Syria's revolt against President Bashar al-Assad erupted 18 months ago.

"We have all necessary plans in place to protect and defend Turkey if necessary," Secretary-General Anders Fogh Rasmussen told reporters before a meeting of the alliance's defence ministers in Brussels.

On Monday, Turkish President Abdullah Gul said the "worst-case scenarios" were now playing out in Syria and that Turkey would do everything necessary to protect itself.

Gul said that the violence in Turkey's southern neighbour, where a revolt against Assad has evolved into a civil war that threatens to draw in regional powers, could not go on indefinitely.

UN Secretary-General Ban Ki-moon said on Monday the escalation of the conflict along the Turkey-Syria border and the impact of the crisis on Lebanon were "extremely dangerous".

The Turkish army fired back on Monday for a sixth day after a shell from Syria flew over the border and has bolstered its presence along the 900-km (560-mile) frontier in recent days.

Rasmussen commended the Turkish government for its restraint, saying he hoped the parties would avoid an escalation of the crisis.

"Obviously Turkey has a right to defend herself within international law," he said. "I would add to that that obviously Turkey can rely on Nato solidarity."

Turkey joined Nato in 1952. – Reuters