The Islamic State group has lost 22 percent of the territory it held at the start of 2015, military analysts IHS Jane's said Wednesday, March 16 as U.S. and Russian air strikes have helped the jihadists' opponents advance, AFP said.

IS controls swathes of territory in Iraq and Syria but lost 14 percent of it last year and a further eight percent this year, according to the IHS Conflict Monitor.

IHS Jane's said the jihadists controlled 73,440 square kilometres of ground as of Monday, an area equivalent to around half the size of England.

The Syrian government has made gains in the west of the country and is now five kilometres outside the ancient city of Palmyra, which was overrun by IS fighters in mid-2015.

"The Islamic State is increasingly isolated, and being perceived as in decline," said IHS senior analyst Columb Strack. He said the group's reversal of fortunes "plays into the hands" of its main rival, Al-Qaeda's Syrian affiliate the Al-Nusra Front.

"Isolation and further military defeats will make it harder for the Islamic State to attract new recruits to Syria from the pool of foreign jihadis," said Strack, according to AFP.

Following the loss of the strategically important town of Tal Abyad on Syria's border with Turkey last year, IHS began to register signs that IS was struggling financially, it said.