* Neighbours have repeatedly exchanged fire
* Rasmussen says have plans in place to defend Turkey
* Ban says escalation "extremely dangerous"
(Adds quotes from U.S. official, NATO head)
By Sebastian Moffett and David Alexander
BRUSSELS, Oct 9 (Reuters) - NATO has plans in place to defend Turkey against attack from Syria, and will aim to provide assistance if Ankara asks for it, senior officials said on Tuesday.
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.
A senior U.S. defence official said the alliance would likely react if Turkey made a request for assistance.
"The allies would have to hear what Turkey says and decide what kind of assistance the alliance should bring," he said. "We engage with Turkey to make sure that should the time come where Turkey needs help we're able to do what we can."
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.
Turkey would have to request military assistance under NATO's collective defence provisions, known as Article 5 - in which an attack on one member is viewed as an attack on all.
It has only been used once before, after the Sept. 11 attacks in 2001, and would involve helping Turkey, not necessarily intervening in Syria.
The head of NATO, confirming that the 28-member military alliance had plans in place to defend Turkey, would not say what these were.
"Taking into account the situation at our southeastern border, we have taken the steps necessary to make sure that we have all plans in place to protect and defend Turkey," Secretary-General Anders Fogh Rasmussen told reporters during a meeting of the alliance's defence ministers in Brussels. "But I think you understand why we can't go into details."
U.N. 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."
He added that it would be surprising if NATO did not have plans to defend and protect all its allies.
Turkey joined NATO - which was set up to defend the territory of its allies - in 1952.
Russia, which has blocked U.N. Security Council resolutions that would allow greater western intervention in Syria, warned NATO not to intervene after the Syrian shelling.
"We expect NATO not to use this tragic incident as a pretext for interfering in the Syrian conflict," Russia's Acting Permanent Representative to NATO Nikolay Korchunov said in a written reply to questions from Reuters.
"Our only hope is that this tragedy is not used as casus belli and won't be copied to provoke unilateral reaction from Turkey," he said.
U.N. special envoy Lakhdar Brahimi will visit Syria soon to try to persuade Bashar al-Assad's government to call an immediate ceasefire, Ban said on Tuesday. Efforts by Brahimi's predecessor, Kofi Annan, to engineer a truce collapsed within days.
(Additional reporting by Adrian Croft; Editing by Alison Williams)
Keywords: NATO TURKEY/RASMUSSEN