UPDATE 2-NATO aims to help defend Turkey if requested-U.S. official

* 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 todefend Turkey against attack from Syria, and will aim to provideassistance if Ankara asks for it, senior officials said onTuesday.

NATO ambassadors threw their support behind Turkey in anemergency meeting last week after Syrian shells struck a bordertown in Turkey killing five civilians.

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

A senior U.S. defence official said the alliance wouldlikely react if Turkey made a request for assistance.

"The allies would have to hear what Turkey says and decidewhat kind of assistance the alliance should bring," he said. "Weengage with Turkey to make sure that should the time come whereTurkey 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 thatTurkey 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 thatthreatens to draw in regional powers, could not go onindefinitely.

Turkey would have to request military assistance underNATO's collective defence provisions, known as Article 5 - inwhich an attack on one member is viewed as an attack on all.

It has only been used once before, after the Sept. 11attacks in 2001, and would involve helping Turkey, notnecessarily intervening in Syria.

The head of NATO, confirming that the 28-member militaryalliance had plans in place to defend Turkey, would not say whatthese were.

"Taking into account the situation at our southeasternborder, we have taken the steps necessary to make sure that wehave all plans in place to protect and defend Turkey,"Secretary-General Anders Fogh Rasmussen told reporters during ameeting of the alliance's defence ministers in Brussels. "But Ithink you understand why we can't go into details."


U.N. Secretary-General Ban Ki-moon said on Monday theescalation of the conflict along the Turkey-Syria border and theimpact of the crisis on Lebanon were "extremely dangerous".

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

Rasmussen commended the Turkish government for itsrestraint, saying he hoped the parties would avoid an escalationof the crisis.

"Obviously Turkey has a right to defend herself withininternational law," he said. "I would add to that that obviouslyTurkey can rely on NATO solidarity."

He added that it would be surprising if NATO did not haveplans to defend and protect all its allies.

Turkey joined NATO - which was set up to defend theterritory of its allies - in 1952.

Russia, which has blocked U.N. Security Council resolutionsthat would allow greater western intervention in Syria, warnedNATO not to intervene after the Syrian shelling.

"We expect NATO not to use this tragic incident as a pretextfor interfering in the Syrian conflict," Russia's ActingPermanent Representative to NATO Nikolay Korchunov said in awritten reply to questions from Reuters.

"Our only hope is that this tragedy is not used as casusbelli and won't be copied to provoke unilateral reaction fromTurkey," he said.

U.N. special envoy Lakhdar Brahimi will visit Syria soon totry to persuade Bashar al-Assad's government to call animmediate ceasefire, Ban said on Tuesday. Efforts by Brahimi'spredecessor, Kofi Annan, to engineer a truce collapsed withindays.

(Additional reporting by Adrian Croft; Editing by AlisonWilliams)

((sebastian.moffett@thomsonreuters.com)(+32 477 981246)(Reuters Messaging:sebastian.moffett.thomsonreuters.com@reuters.net))