A monitoring group said today that, following its takeover of the ancient city of Palmyra, Isis now controls more than 95,000km² of Syria, equating to more than half of the country's territory.
Forces loyal to the Syrian regime are retreating westward as president Bashar al-Assad seeks to consolidate key sites following rapid gains by Isis and other rebel forces.
The capture of Palmyra, which is listed by UNESCO as a world heritage site and contains well-preserved Roman ruins, was the first time the militants have taken a city directly from regime forces.
Additionally, the army lost its last military base in Idlib province on Tuesday after they were routed by a recently-formed rebel coalition. The loss of Al Mastoumah military camp means that Assad's forces have been almost completely driven out of the northwestern province.
The regime still controls strategically significant stretches of Syria, including most of the M5 highway which connects the capital Damascus with important cities including Homs and Aleppo. The regime is also strong along Syria's Mediterranean border in cities such as Latakia, a traditional heartland of Assad's Alawite sect.
However, according to Lev Yuriditsky, Syria analyst at Tel Aviv-based consultancy Max Security, the regime is losing momentum and will face further strategic losses in the near future.
"They are definitely backpedalling at the moment, they haven't really had any success in a while," says Yuriditsky. "The regime forces are pressured in numerous directions and it's a very hard decision to choose where they are going to fight right now."
In Idlib, government forces have retreated to Ariha, one of the few towns they still hold in the province. However, Yuriditsky says the takeover of Ariha by rebel forces is guaranteed and following that predicts a huge battle in the coming weeks for control of a strategic air base in Deir ez-Zor in eastern Syria.
The airbase represents the regime's connection to the east of the country, and they have been able to defend until now, most recently repelling a previous attack from Isis in March. "Once they lose the air base, they won't have any aerial capabilities in that area," says Yuriditsky.
The loss of Palmyra to Isis was a huge blow to the regime. As well as its historic and cultural value, Palmyra is directly connected by road to Homs, which in turn lies on the M5 highway described as the "de facto border of the regime" by Benjamin Decker, senior intelligence analyst at the Levantine Group.
"For Isis, the capture of Palmyra is very significant and not necessarily for the reasons UNESCO would like to believe. Obviously the destruction of ancient artefacts is a complete catastrophe, but what Isis has been able to do is capture air bases which are used by the regime for air strikes and barrel bombing," says Decker.
The taking of Al Mastoumah by the recently formed Jaysh al-Fateh, a rebel coalition led by the al-Qaeda linked Nusra Front, represents the group's biggest gain since it was set up in March.
It is also a significant development in the civil war, which has now been raging in Syria for four years. The fact that moderate rebel groups are now working alongside militants affiliated to al-Qaeda makes the concept of western training and arming of so-called moderate rebels extremely fraught.
Assad's regime has been heavily reliant upon international support to stay in power, particularly from Iran and Russia. The UN envoy to Syria reportedly told a private Washington meeting that Tehran finances Damascus by as much as $35bn per year. In 2013, Tehran provided a $3.6bn loan to Damascus to buy oil.
Iranian-backed Hezbollah fighters have also been instrumental in key battles for the regime, most recently in the Qalamoun region on the Syrian-Lebanese border.
A senior adviser to Iran's supreme leader Ayatollah Ali Khamenei said on Tuesday that Tehran would continue to offer whatever support was necessary to Assad after meeting with the Syrian leader in Damascus.
Moscow has consistently defended the Syrian government on the international stage, including using its veto to block an investigation into the conflict by the International Criminal Court.
More than 210,000 people are estimated to have died in the Syrian conflict, while the UN says that almost four million Syrians have been made refugees by fighting throughout the country.Try Newsweek: Subscription offers