Turkey's allies know it is playing a double game with its twin onslaught against Kurdish rebels and the Islamic State group, but are turning a blind eye to keep NATO's only Muslim member on side, analysts said.

The very public show of solidarity for Turkey's fight against "terrorism" at an emergency NATO meeting on Tuesday hid the discomfort some allies feel about President Recep Tayyip Erdogan's strategy.

Suspicions are swirling that Erdogan's sudden embrace of the US-led campaign against IS in Syria and Iraq simply provides the cover to pound Kurdish rebels, viewed in the West as a bulwark against the jihadists.

Western capitals had "massive mistrust" in the motives of Turkey, which under the authoritarian and Islamist-rooted Erdogan has been accused of turning a blind eye of its own to IS, said Ege Seckin, Turkey analyst at IHS Country Risk.

"The member states are fully aware that the Turkish priority is an attack on the Kurds -- to be more specific, the prevention of a contiguous Kurdish entity in northern Syria," Seckin told AFP.

"Attacking IS is more a concession given to the United States."

Michael Stephens, head of the British Royal United Services Institute's (RUSI) centre in Qatar, said the priority for Turkey's NATO allies was hitting Islamic State militants.

"The key here for the allies is not to affect the operations against IS in Syria -- if they are not affected then to some extent the PKK and Turkey can work their own process out," he said.

- 'Good' Kurds, 'bad' Kurds -

Turkey launched strikes against IS last week following a devastating suicide bombing in the largely Kurdish border town of Suruc.

Washington and Ankara shortly afterwards announced a pact to create an "IS-free zone" in northern Syria, and Turkey gave the US approval to use the strategic Incirlik air base near the Syrian border for anti-IS raids.

But Erdogan rapidly extended the campaign to include the rebel Kurdistan Workers' Party (PKK) in Turkey and northern Iraq after the militants claimed a series of attacks on Turkish security forces in revenge for the Suruc blast.

A host of key figures, including French President Francois Hollande, NATO chief Jens Stoltenberg, and Germany's defence minister have urged Ankara to show restraint and restart the peace process to end decades of conflict between the Kurds and Ankara.

Analysts said Turkey's NATO allies were making a critical, and pragmatic, distinction between the PKK and other Kurdish groups.

These include the Kurdish People's Protection Units (YPG) in northern Syria whose men have been fighting IS with the backing of US air strikes for months, particularly in the flashpoint town of Kobane.

RUSI's Stephens said Ankara "backed off right away" when the Syrian Kurds accused Turkish forces of shelling them over the weekend and it was "clear the Americans sent out the message that these guys are not to be touched".

- 'Can't break with Turkey' -

For Washington, the "number one issue is IS" and it was important to have Turkey on side for that, said Ian Lesser, senior director, foreign and security policy with the German Marshall Fund.

He said that shifting allegiances in the Syrian conflict, combined with historical tensions, meant that it was a "highly nuanced, very varied scene."

"IS is always a murky issue and Turkey has played a less than transparent game," while most of Europe considers that the PKK is a terrorist group but "the conundrum is not easily resolved," Lesser added.

Turkey had made a bargain with its allies to pursue its own objectives, said Baghdad University professor and analyst Ihsan al-Shammari.

"The strikes against IS are probably what Turkey has to pay the Americans to have a free rein in hunting down the Kurds," he said.

In the end, NATO's need to keep Turkey in the fold as the alliance faces multiple threats to the south, the Middle East and in the east from an assertive Russia, looks set to trump any other concerns.

"Turkey, even if you disagree with their policies, it is one of the last remaining allies in the region," said Seckin.

"You can't break relations with Turkey over this. Ultimately it's about restraint -- I know that it sounds like a weak policy, but when two friends fight, what else can you do?" added RUSI's Stephens.

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IHS