Islamic State is preparing itself for the potential breakdown of the Syrian government by taking key cities.

The militant group recently took the Iraqi city of Ramadi, to increase their territorial holdings to roughly a third of the troubled nation.

They now control the Syrian city of Palmyra, home to many historic and ancient artefacts.

Otago University's Professor of Middle Eastern Politics William Harris says it could be a tactical move by the insurgents.

He says taking the city has placed ISIS in a favourable position to strike further into Homs and Damascus.

"ISIS is, like others, positioning itself for a possible cracking apart of the Syrian regime, which would be a political earthquake in Middle Eastern and global terms."

Homs is a key city that has been heavily damaged during the Syrian civil war, and Damascus is the capital of the Bashar al-Assad led regime.

Assad's forces are fighting on multiple fronts against various political and Islamist rebel groups, which may have been a factor in the loss of Palmyra, according to William Harris.

"It seems to be a mixture of the regime deciding it should fall back due to limited resources, and an actual defeat by the ISIS people."