Set on the Euphrates River inside Syria, a centuries-old tomb complex considered Turkish territory has become a new potential flashpoint in the conflict with jihadists that could be crucial in determining Ankara's future policy.

For decades, few paid much attention to the quirk of history that had left the tiny scrap of land inside Syria that houses the tomb of Suleyman Shah -- the grandfather of the founder of the Ottoman Empire Osman I -- as sovereign Turkish territory.

But with Islamic State (IS) jihadists controlling swathes of northern Syria and now bearing down on the tomb itself, the exclave has become a critical factor in Turkey's sometimes ambiguous, but now changing, role in the conflict.

The tomb of Suleyman Shah, around 25 kilometres (15 miles) south of the Turkish border, is considered Turkish territory under the 1921 Treaty of Ankara between the Turkish authorities and France, which then controlled French-mandated Syria.

Turkey has maintained a continuous military presence to guard the tomb but the position of the few dozen troops stationed there has become all the more precarious amid the IS militants' advance.

President Recep Tayyip Erdogan has indicated that Turkey would strike back if there was an attack on the tomb, saying its answer to any aggression would be "clear".

Furthermore, if the tomb was attacked NATO member Turkey could in theory invoke Article V of the alliance's treaty that an attack on one member is an attack on them all.

- 'You are not alone' -

In a sign of the seriousness of the situation, Turkey's top general Necdet Ozel on Thursday issued a special message for the soldiers on duty at the tomb, saying the army would come to help them whenever they needed.

"Don't forget that you are not alone. Don't forget that 76 million of our citizens are committed to standing behind you," he told the troops guarding Turkey's only overseas territory.

"Feel confident that our armed forces will be there for you the moment we hear a single word from you," he added.

The tomb was for years guarded largely by conscripts but in March this year Turkey reportedly sent a special mission of 60 elite special forces who remain on duty there to this day.

The Turkish leadership has vehemently rejected a report in the pro-government Yeni Safak daily that the tomb had been surrounded by some 1,100 IS militants and the Turkish soldiers taken hostage.

But Defence Minister Ismet Yilmaz acknowledged in parliament Thursday that IS was advancing on Suleyman Shah, describing its progress as "a threat to our national security."

Some press reports have suggested the general staff was unhappy that Prime Minister Ahmet Davutoglu did not allow the evacuation of the troops. However not a flicker of dissent has emerged from the military in public.

Turkish media have said the armed forces have developed a plan to repel any IS attack within 15 minutes through attack helicopters.

Turkish F-16 jets are already carrying out reconnaissance missions in the border area around the tomb, reports said.

- 'Likely target for retaliation' -

The issue of the tomb has become all the more burning as Turkey considers taking a more active role in the fight against IS, after exasperating the West for months with a decidedly reticent policy.

This could make the Turkish contingent highly vulnerable if IS seeks revenge against Ankara for taking part in the US-led coalition against it.

"In the light of Turkey's shifting policy, the threat to the tomb has become more palpable," the Istanbul-based Centre for Economics and Foreign Policy studies said in a report Wednesday.

"If Ankara decides to take part in the coalition effort, the tomb of Suleyman Shah would be one of the likely targets for IS retaliation," it added.

Turkey could use its air forces to rescue the troops or strike against jihadists attacking the tomb, although they could be at risk to assaults from shoulder-launched missiles.

"Hence Ankara should be cautious in drafting and carrying out its potential military responses, as it may quickly be drawn further into the calamity at its doorstep," it said.