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By Daren Butler
ISTANBUL, Oct 16 (Reuters) - Turkey's army said on Wednesday it had fired on al Qaeda-linked militants in northern Syria after a stray mortar bomb hit Turkish territory, highlighting the growing threat from radical groups over its southern border.
Fighters from the al Qaeda-affiliated Islamic State of Iraq and the Levant (ISIL) have seized territory in parts of Syria near the Turkish border in recent weeks, leaving the already fragile frontier even more vulnerable.
Turkey's general staff said the army had fired four artillery shells on Tuesday at ISIL positions around Azaz, a strategic area around 5 km (3 miles) from the frontier seized by the al Qaeda-linked fighters last month.
Turkey has repeatedly carried out such retaliatory action when fire from Syria has hit its soil in the past but this appeared to be the first time it had targeted ISIL militants.
The move comes as Turkey, a key backer of the Syrian opposition, faces accusations from Western allies that it has allowed arms and foreign fighters to cross into northern Syria, facilitating the rise of radical groups, a charge it denies.
Turkey has maintained an open-door policy throughout Syria's two-and-a-half-year conflict, sheltering half a million refugees and allowing the Syrian opposition to organise on its territory. But it denies arming the rebels.
The military said a mortar bomb was fired from the Azaz/Parsa mountain area of northern Syria at around 1:30 p.m. (1030 GMT) on Tuesday and landed, unexploded, 450 metres (1,500 feet) east of a military border post at Demirisik in Kilis province.
"In response to this situation, four shells were fired by two Firtina (Storm) howitzers at positions of the Islamic State in Iraq and the Levant in the Azaz/Parsa mountain," it said.
It was not clear if the shells caused any damage.
Fighting around Azaz between ISIL and rebels from the local Northern Storm brigade prompted Turkey to briefly close its border crossing last month, a lifeline for Syria's rebel-held areas which allowed humanitarian aid in and refugees out.
Divisions among the rebels have hurt their fight against President Bashar al-Assad's better equipped and organised forces. Tensions have been rooted partly in conflicting ideologies, but more often in disputes over resources, land and the spoils of war. (Writing by Nick Tattersall; Editing by Andrew Roche)