An infidel horde flying 80 banners meets a Muslim army at the Syrian town of Dabiq in an apocalyptic battle. The Muslims are decimated but ultimately prevail, ushering in the end of days.

This ancient Sunni Muslim prophecy -- mentioned in canonical accounts of the Prophet Mohammed's sayings -- has become a rallying cry for Islamic State jihadists in Iraq and Syria, especially since they seized Dabiq in August.

The town itself has negligible military value compared with the strategic IS-controlled cities of Raqa in Syria and Mosul in Iraq.

But as IS jihadists come under a US-led aerial onslaught to stop their advance, its importance as a symbol has become clear.

"It raises morale," said Shadi Hamid, a fellow at the Brookings Institute. "It is fair to assume that the vast majority of (IS) fighters believe in this type of talk."

Among IS supporters on social media, Dabiq has become a byword for a struggle against the West, with Washington and its allies bombing jihadists portrayed as modern-day Crusaders.

IS has even named its official magazine simply "Dabiq".

"The lions of Islam have raised the banner of the Caliphate in Dabiq," one Tunisian IS supporter wrote recently on Twitter. "Now they await the arrival of the Crusader army."

The prophecy has been passed down in different versions, but in all cases it features a great battle between a Muslim army and the forces of non-believers.

Recent weeks have seen IS supporters interpreting a wide range of events as further evidence of its truth.

Some keep a close count of the US-led coalition's members -- now at more than 60 countries -- in anticipation of when the prophecy's "80 banners" are reached.

Others have interpreted comments by top US General Martin Dempsey on the possible need for ground forces as a signal of the foretold battle, writing on Twitter using the hashtag: "It is Dabiq, by God."

- The foretold 'caliphate' -

One IS supporter wrote on Twitter: "When you despair of your air power, you will find us waiting in Dabiq."

Some versions of the prophecy mention the Muslim army moving on after the great battle to take Constantinople, the former capital of the Christian Byzantines and present-day Istanbul.

When Turkey decided last week to join the fight against the jihadists, that too was greeted as an omen by some IS supporters.

Prophecy has played a role in the movement's ideology since its early days as Al-Qaeda in Iraq under Abu Musab al-Zarqawi.

Before Zarqawi was killed in Iraq in a US airstrike in 2006 -- and long before his movement evolved into IS -- he was already referring to the epic battle in Dabiq.

"The spark has been ignited in Iraq, and its flames will grow until they burn the Crusader armies in Dabiq," he once said.

When IS earlier this year seized large parts of Iraq and proclaimed its current leader Abu Bakr al-Baghdadi as "caliph", it again turned to prophecy to rally supporters to its cause.

One of Mohammed's prophecies is of the rise of a "caliphate on the path of the Prophet" and shortly before the proclamation IS spokesman Abu Mohamed al-Adnani pledged that "God's promise" was imminent.

Some IS supporters on social media have rallied behind Baghdadi as the foretold caliph, but have been more circumspect regarding the advent of the battle in Dabiq.

"God knows, maybe. The hadith (prophetic saying) says they will fight you under 80 banners. At the moment there's 60 banners I think," said a British IS fighter in Syria in a private message to AFP on Twitter.

"It could happen now; it could happen in the future," said Anjem Choudary, a radical British Islamist preacher who has expressed support for IS.

"I don't think any Muslim strives to bring it about," he said of the battle in Dabiq.

Yet some IS supporters are convinced the destined battle is near.

"Dabiq will happen for certain. The US and its allies will descend on Syria once they see that the air campaign has failed. That is a promise by God and his Messenger," wrote one on Twitter.