ISIS is close enough to the Syrian President's seat of power that it could circumvent the Syrian regime's security belts and enter central Damascus via tunnels, a Middle East expert has warned.
Isis is now just five kilometres away from President Bashar al-Assad's Syrian Presidential Palace after militants invaded the sprawling Palestinian refugee camp in Damascus, which has over 18,000 refugees living within its walls.
Omar Ashour, an Associate Fellow for the Chatham House think tank and a Senior Lecturer in Security Studies at the University of Exeter, said the situation in Damascus is now "critical" and could explode into a prolonged war within Yarmouk, depending upon the next steps taken by the extremist group and President Assad's regime.
In an unusual alliance, the Syrian government has offered military support to Palestinian fighters battling Isis in order to flush the group out of the camp.
The Deputy Foreign Minister Faisal Meqdad met with a delegation from the Palestine Liberation Organisation (PLO) this week.
"Syria and the PLO are determined to fight terrorism, which has reached Palestinian camps in Syria, notably Yarmouk," he said after delegates met. This offer is one that has reportedly been accepted by Palestinian factions.
Activists say regime forces have already started "indiscriminately" bombing the refugee camp and neighbouring Hajar al-Aswad in response to the invasion by Isis.
However, Mr Ashour told The Independent the coming weeks could see the Syrian military take a strategic step back from the situation and allow Isis to fill the vacuum created in its absence.
"Isis would do part of the job which is fighting other Islamist and revolutionary groups, launching an attrition war," he said.
"The regime would then finish them off afterwards by aerial bombardment. But if it got out of hand and Isis was able to capitalise on it, then the regime would have made a miscalculation. It would be a very close deal."
Yarmouk was infiltrated by militants from the al-Qaeda affiliate Jabhat al-Nusra and other militant groups over a year ago. Mr Ashour believes that what will follow will be a prolonged war between groups for control of the camp, which would weaken everyone inside it.
He warned that in the unlikely event Isis does form an alliance with Islamist groups inside instead of fighting and focused their attention on entering central Damascus, they would present a very serious threat.
"I think what is more likely to happen is that Isis will keep fighting in Yarmouk. Their approach is about centralisation; if there is no centralised command and control under the leadership, then they will keep fighting. It is more likely that they will drain the other forces, then achieve victory and achieve centralisation. This is what the regime is calculating.
"If they do not stop fighting in the next few days, we are looking at a prolonged attrition war with no side winning. Isis will probably send for reinforcement, the southern front and some of the groups there may become involved and it could galvanise into a full scale war.
"There are a string of shops and streets just a few miles away in central Damascus - if they dug a long tunnel they would get there without a problem.
"But, if Isis do decide to form a truce with all of the others then we could see more of a focus on the regime - although it has already demonstrated how opposed to truces they are. If there is a significant fighting force in Yarmouk and it was able to coordinate together, I think this would be a very serious threat. Remember, one or two tunnels and they could be into central Damascus."
Elijah J Magnier, the chief international correspondent for Al-Rai Media, has been reporting on the situation from close to Yarmouk in Damascus. He said the regime is already responding to the threat posed by Isis by bombarding Yarmouk and Isis positions in Hajar al-Aswad with devastating 'volcano missiles'.
"Assad is not happy about having Isis south of Damascus and just kilometres from the city centre," he said.
"The Syrian Army is already stretched everywhere in Syria and he wouldn't like to have another new front. Therefore, the collaboration of all Palestinian groups, supporters by the Syrian Army, is needed to defeat both Isis and Nusra in the camp. It is a dangerous situation for all.
He agreed that a lasting coalition between Isis and Nusra within the walls of the camp would pose a grave danger to Assad, but believed this outcome was highly unlikely.
"Nusra can close an eye to Isis but it will not join them now. There are many common fronts between Nusra and Syrian rebels in Daraa, Idlib, and Aleppo. Nusra won't risk this alliance to join an unpredictable Isis."
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