The US-led coalition has made Iraqi and Syrian Kurdish forces primary allies against the Islamic State jihadist group, but over-reliance on the Kurds carries risks, analysts warn.

As the world seeks to turn up the heat on IS, some of the West's main partners on the ground are the peshmerga forces from Iraq's autonomous Kurdish region and the Kurdish People's Protection Units (YPG) in Syria.

The first soldiers officially deployed by the United States in Syria arrived last week in the north to train the YPG, a group which has close ties to Turkey's terror-listed Kurdistan Workers' Party (PKK) rebels but which has also notched up significant military successes against IS.

In the aftermath of the deadly November 13 attacks claimed by IS in Paris, French Prime Minister Manuel Valls emphasised the need to support Kurdish forces on the ground.

After IS took over swathes of Iraq in 2014, Washington launched air strikes alongside a programme to train and equip local forces.

The US "picked the Iraqi Kurds because they were strategic partners during the 2003 invasion and were, at least in their eyes, the most trustworthy," said Maria Fantappie, Iraq senior analyst with International Crisis Group.

More than a year on, multi-million-dollar attempts to groom Sunni Arab forces in both Iraq and Syria have yielded limited results at best and failed to sabotage IS' self-proclaimed "caliphate".

Attacks in France, elections in the US and a migrant crisis across Europe converge to up public pressure for swift and decisive action against the increasingly global threat of IS.

- Bargaining chips -

Kurdish forces are among the most skilled, organised and determined to battle IS in the region.

But analysts warn military action should be matched with political planning for the post-IS era in Iraq and Syria, and that relying too heavily on the Kurds could backfire.

The lack of a roadmap addressing Kurdish statehood aspirations is an incentive for groups to secure as many future bargaining chips as possible by winning military brownie points now.

Fantappie said that explains why the YPG might be prepared to push beyond Kurdish areas and take part in an offensive to recapture the IS hub of Raqa, an almost entirely Arab city.

"Definitely this is on their mind, especially for the YPG, which strives to gain international recognition," she said.

In neighbouring Iraq, forces loyal to the regional Kurdish president Massud Barzani last month retook the town of Sinjar, the main hub of Iraq's Yazidi minority.

Before IS swept across Iraq last year, it was under Baghdad's authority, not part of the autonomous Kurdish region, but Barzani is now pushing plans to maintain control of the area.

Barzani "effectively announced Sinjar's annexation into the Iraqi Kurdistan region," Patrick Martin, Iraq researcher at the Institute for the Study of War, said.

"There have been no indications that Kurdish fighters are prepared to hand control of the district to the Iraqi federal government," he said.

- 'Imbalanced relationship' -

Michael Knights, a fellow at the Washington Institute, which focuses on US policy in the Near East, said any operation to free Iraq's second city Mosul from IS would be headquartered on Kurdish real estate.

"Until (IS) is removed from Mosul, the Kurds will remain a key ally. Thereafter, their future is much harder to gauge," he said.

Knights said that Kurdish expansion was already close to peaking in Iraq and would be limited by a negative reaction from Baghdad, which also receives help from the coalition against IS.

The same limitation applies to Syria, where too much consolidation of Kurdish influence in the north would not sit well with NATO member Turkey.

Even where the coalition is trying to foster Kurdish-Arab alliances against the jihadists, the relationship is tilted in the Kurds' favour, Fantappie said.

"By picking the Kurds as strategic allies, you have created an imbalanced relationship between the Kurds and the other communities living with the Kurds," she said.

In northern Syria, the Syrian Democratic Forces alliance brings together the YPG and Arab forces, but the Kurds have direct access to funds and weapons while their partners do not, she said.

"And this is dangerous because this military support can have unintended consequences..., can redraw borders within those countries and create the premises for future conflicts and tensions between the Kurds and their neighbours."