The conflict in Syria has seen the rise to prominence of an Islamist militant group called the Islamic State of Iraq and the Levant (ISIS).

ISIS - the final 'S' refers to al-Sham, an Arabic word for the 'fertile crescent' region in the eastern Mediterranean - has become a leading force in anti-government attacks in Syria and Iraq.

In doing so its fighters have taken advantage of a power vacuum, generated in the case of Iraq by the overthrow of Saddam Hussein's dictatorship in the 2003 US-led invasion, and the fractured state of rebel forces.

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The Al Qaeda-linked jihadist group has mounted hundreds of attacks in Syria since 2011 and played a leading role in seizing a military air base in the northern city of Aleppo last August.

It also claims responsibility for numerous operations in Iraq: Islamists from the group have now seized control of Fallujah and most of Ramadi, the main cities in the Sunni Muslim-dominated province of Anbar.

The group's avowed aim is to set up a new Sunni state straddling the border areas linking Iraq and Syria, and based on sharia law.

Genesis of a terror group: When Iraq's Al Qaeda met Syrian Islamist rebels

With the withdrawal of US forces from Iraq in 2011, the Sunni jihadist group Al Qaeda in Iraq (AQI) - which had been a key force in the anti-American insurgency - not only increased attacks there but expanded its reach into neighbouring Syria.

Rebranding itself as ISIS, the group recruited members of rebel groups opposing the regime of president Bashar al-Assad, including those from the Syrian jihadist group, the Jabhat al-Nusra Front.

Al-Nusra rose to early prominence in the Syria conflict as one of a number of rebel groups opposing the Assad regime, and quickly gained popularity with brazen attacks that it said were a response to alleged atrocities committed by Syrian government forces.

The US fears that Al-Nusra is using the Syrian conflict to further its jihadist ideas and goals for an Islamic state, raising fears that Syria will likely become a hotspot for Al Qaeda activity.

Washington has designated the group a foreign terrorist organisation, citing its links to Al Qaeda in Iraq.

ISIS fighter mount a show of force in the Syrian town of Tel Abyad on January 2 ( Reuters: Yaser Al-Khodor )

ISIS v Al-Nusra: rival groups sharing some common goals

Experts differ on the topic of cooperation between ISIS and Al-Nusra.

Aaron Y Zelin, a researcher at Washington DC-based think tank The Washington Institute, says that in 2011 ISIS leader Abu Bakr al-Baghdadi tried to merge with Al-Nusra, which at the time was "one of the (Syrian) opposition's best fighting forces".

The legacy of war Al Qaeda, which had no presence in Iraq prior to the US invasion, now has a niche there, writes Amin Saikal.

Al-Nusra's leader Abu Muhammad al-Jawlani ultimately rejected the merger, though not before many Syrian jihadists had left his group for ISIS.

ISIS has since emerged as the more dominant group, attracting a large number of Syrian and foreign fighters.

ISIS is also known as the "Islamic State of Iraq and Syria" and the "Islamic State of Iraq and al-Sham", al-Sham being a historic Arabic name for the "Fertile Crescent" area and referring to a region comprising areas of modern Iraq, Syria, Israel, the Palestinian Territories and Lebanon.

Levant, a Western term, refers to the same geographic area of the Middle East.

'One of the world's most deadly terrorist organisations'

The Australian government has added ISIS to its list of terrorist organisations, replacing its listing for Al Qaeda in Iraq to reflect the "expansion of the organisation's operating area to include Syria".

Attorney-General George Brandis described ISIS as "one of the world's most deadly and active terrorist organisations", saying the group - which includes many foreign fighters, including Westerners - conducts "frequent and often indiscriminate attacks including the targeting of public gatherings to maximise casualties".

Senator Brandis said the group's establishment of independent operations in Syria had resulted in the defection of some Al-Nusra members to ISIS.

Could northern Syria become the new Afghanistan?

British journalist Owen Bennett Jones points to the "remarkable" resurgence of Al Qaeda over the last three years, led by groups such as ISIS and the Al-Nusra Front.

Northern Syria, he writes, "may well come to play the role Afghanistan once played as a place for jihadis to train before they are deployed back in their home countries".

ISIS is fighting for Islamic rule not only in Syria but in Lebanon, Israel and the Occupied Territories, he warns, and is responsible for as many as 68 car bombings a month in Iraq.

Its operations to retake Ramadi and Fallujah have sent residents fleeing to nearby villages and cities to escape heavy artillery bombardment and jet strikes by government troops.

Despite attacks by the Iraqi air force, and prime minister Nouri al-Maliki's threat to retake the city, ISIS fighters have been seen using police and government vehicles for patrols in Fallujah, some flying a black flag associated with Al Qaeda from the vehicles.

Fears that ISIS could mount a terrorist operation on the scale of Al Qaeda under Osama bin Laden are premature, according to Amin Saikal, director of the Centre for Arab and Islamic Studies at the Australian National University.

However, the group does pose a threat, he says.

"Al Qaeda was always able to engage in terrorist activities, to hit a target which can bring them a lot of publicity. I think ISIS will be similar," he said.

And were the group to succeed in its goal of establishing an Islamic state in Iraq and Syria, it could "put them in a position to challenge, for example, the occupation of Palestinian lands", he added.

ABC Middle East correspondent Matt Brown reports that the jihad in Syria is "drawing men willing to fight and die from one of the most isolated places in the Middle East", the Gaza Strip, even though it is entirely surrounded by Israeli and Egyptian forces.

The development has Gaza-based political analyst Mkhaimar Abu Sada concerned that the Syrian war is fanning the flames of radicalism in the Palestinian territories.

"Bringing back some Salafis (Sunni Muslims) who are trained in warfare and explosives, that might pose a real threat to the stability of the security situation here in the Gaza Strip," he said.

Sorry, this video has expired A dramatic development in Syria's civil war is the rise of foreign jihadists, including Palestinians from Gaza.

Professor Saikal, meantime, said it remains to be seen whether ISIS, despite its gains in Fallujah and Ramadi, can withstand a military onslaught by the US-backed Iraqi government.

Hearts and minds: Learning from past jihadi atrocities

Mr Zelin, who researches how jihadist groups operate in the wake of the Arab uprisings, writes that ISIS "bolstered its growing reputation as a key player in the jihad against the Syrian regime" with the takeover of Minakh air base in Aleppo, among other operations.

However, he writes, a large part of the group's success lies in its efforts to cultivate support among ordinary Syrians and "avoid repeating the mistakes that its predecessors made in Iraq", namely the beheadings and other brutal activities carried out by Al Qaeda in Iraq in the early stages of the sectarian war there.

"Besides light-hearted activities aimed at endearing themselves to the people, ISIS members have also provided aid to civilian protestors in Damascus, free medical services to locals in Jarabulus, bags of food to the needy in rural Aleppo, and below-market fuel to residents in Deir al-Zour governorate. These materials have been branded with the group's black flag, illustrating that ISIS has significant organizational and financial resources as well as a clear intent to publicise its charitable aims."

Such efforts indicate that ISIS is "attempting to lay the groundwork for a future Islamic state by gradually socialising Syrians to the concept", he writes.

A Syrian rebel fighter guards a street to prevent members of ISIS from entering Idlib on January 6, 2014. ( Reuters: Fadi Mashan )

Ruthless 'masked army' blamed for kidnappings, disappearances, torture of opponents

However, Islamist rebels themselves have accused ISIS of being "worse than the Assad regime", after the group was blamed for the kidnapping and killing of Hussein al-Suleiman, a physician who was also a commander in a rival militia.

Lebanon's Daily Star newspaper cited the Islamic Front militia as saying that Suleiman was arrested after he went to meet with an ISIS delegation in order to settle a dispute that arose in a village near Aleppo.

"They kidnapped him and tortured him, and then killed him and disfigured his corpse, in a way unknown to the Syrian people prior to the revolution, even when it came to the branches of the criminal Assad regime’s security bodies," a statement from the group said.

It warned that such tactics risked "internal fighting, in which the Syrian revolution will be the first loser".

German newspaper Der Spiegel reports that the group has kidnapped hundreds of people, including activists, politicians, Christian priests and several foreign journalists, adding that "anyone who opposes the ISIS fighters, or who is simply considered an unbeliever, disappears".

It cites an engineer who fled Syria after threats he said he received from the group as saying: "We call them the Army of Masks, because their men rarely show their faces. They dress in black, with their faces covered."

Further:

Few dare to stand up to the masked army. When a convoy of ISIS pickup trucks mounted with machine guns rolled into the town of Turmanin in late November, not a shot was fired.

Drug smuggling, arms trafficking and oil fields keep money flowing

Despite AQI presenting itself as a paragon of strict Islamic virtue, the bulk of ISIS's financing, experts say, comes from illegal black market activities in Iraq, including robbery, arms trafficking, kidnapping and extortion, and even drug smuggling.

In the northern Iraq city of Mosul ISIS nets upwards of $8 million a month by extorting taxes from local businesses, according to the US-based Council on Foreign Relations (CFR).

Meanwhile, ISIS's spread across the northern and eastern provinces of Syria bordering Iraq and Turkey has enabled it to take control of Syria's oilfields, the Economist magazine points out.