The Islamic State group has suffered "devastating" blows in Syria's Kobane and on several Iraqi fronts, but analysts warn such victories in the fight against the jihadists cannot be replicated everywhere.

Kurdish fighters backed by US-led airstrikes this week ousted IS from most of Kobane, after a four-month battle whose symbolic importance had far outgrown the small Syrian town's military value.

Simultaneously, Iraqi forces flushed the jihadists out of their last urban bastion in the eastern province of Diyala, further shrinking the borders of their self-proclaimed "caliphate".

"Kobane shows that intense air strikes concentrated in a small space can succeed in containing IS," said Aymenn Jawad al-Tamimi, a Shillman-Ginsburg fellow at the Philadelphia-based Middle East Forum.

"The air strikes were devastating. IS lost a lot of people in Kobane and they're not even trying to spin it," said Patrick Skinner, an analyst with the Soufan Group intelligence consultancy.

In a rare audio message released on Monday, IS spokesman and top leader Abu Mohamed al-Adnani made no reference to Kobane, which both sides had made the nexus of their military efforts.

According to observers, the jihadists lost around 1,200 fighters in the battle of Kobane and some US officials have said that American-led airstrikes killed 6,000 jihadists since the air war started in August.

Over the past month, Iraqi Kurdish forces have also scored significant victories, cutting the group's main supply lines between their hub of Mosul and the Syrian border.

While the noose is tightening on Iraq's second city -- IS' largest urban stronghold -- the capital Baghdad is breathing more comfortably following the "liberation" of Diyala province.

"The group has definitely lost momentum. That goes against (the) notion of continual expansion" it is trying to project, Tamimi said.

"Generally, IS is either losing territory, not making advances at all, or having to recover territory," as is the case in the strategic Iraqi town of Baiji, which the jihadists lost in November.

- 'Strongholds still intact' -

Compounding IS' woes is the resurgence of its jihadist rival in Syria, the Al-Qaeda affiliate Jabhat al-Nusra.

"IS has been under such concentrated pressure -- they're really having some bad times -- while Jabhat al-Nusra has been under the radar," Skinner said.

He said Al-Qaeda had been successfully coopting other rebel outfits and positioning itself to become the most influential group whenever the "moderate" forces being trained by the West are launched into the fray.

However, while Western aerial might played a crucial part, recent victories for anti-IS forces were achieved in areas where the ground offensive was spearheaded by homegrown troops.

In Kobane, the Kurdish People's Protection Units (YPG) did the grunt work, in northern Iraq the Kurdish peshmerga are leading the way and in Diyala Iran-backed Shiite groups were omnipresent.

"IS control over its most important strongholds in Syria and Iraq remains intact and there is a lack of a local military force to challenge IS in places like Mosul," Tamimi said.

The United States and other powers are training the Iraqi army, and Sunni groups opposed to IS are preparing for battle, but analysts said the process required more time.

- 'Air strikes not enough' -

Washington and Baghdad have repeatedly voiced confidence that the current strategy was bearing fruit and would eventually defeat the jihadists.

But some Sunni Arab forces say foreign troops on the ground are necessary to crack IS' biggest strongholds.

"We don't think there is much progress because air strikes are not enough," a senior Middle Eastern official told AFP on condition of anonymity.

Speaking after last week's anti-IS coalition conference in London, the official said his country, which is involved in the air campaign, had offered troops but was turned down by Baghdad.

"We stress that the worst thing that could happen is for Mosul... to be retaken thanks to US raids and a ground offensive led by Shiite militia and Iran with no Sunni involvement."

Skinner pointed out that "it took 10,000 US marines to clear Fallujah (10 years ago), which is a fraction of the size of Mosul."

Figures tell a similar story in Syria.

"It took the YPG on the ground and US-led coalition from the air 112 days to expel IS from Kobane, which covers just six square kilometres," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, a UK-based monitoring group.

"The IS controls some 35 percent of Syrian territory... How long will it take to expel IS from Raqa, Deir Ezzor, and so on?"

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