Turkey is demanding that the main Kurdish militia in Syria be excluded from a US-Russian ceasefire deal expected to take effect at the weekend, President Recep Tayyip Erdogan said Wednesday.

The Kurdish People's Protection Units (YPG) is the main US ally on the ground pushing back the Islamic State extremist group in central and eastern Syria.

But the growth of the YPG and its political wing the PYD, largely at the expense of Islamic State, has sparked concern in Turkey.

"Just like Daesh and al-Nusra are excluded from the ceasefire, the PYD-YPG, which is a terror organization just like them, must be excluded within the scope of the ceasefire," Erdogan said in Ankara, using terms for Islamic State and an al-Qaeda affiliate.

Both Islamic State and al-Nusra Front are on the UN terrorist list and are excluded from the ceasefire deal, planned to come into force on Saturday at midnight (2200 GMT Friday).

Turkey, concerned about Kurdish rebels on its own soil, has pushed, mostly in vain, to have the YPG, a secular-leftist group, equated with al-Qaeda.

Turkey, a strong backer of a range of rebel groups, including some hardline factions, has been shelling the YPG positions in western Syria. The Kurds are taking territory in Aleppo province from rebels weakened by intense Russian airstrikes.

The ceasefire has garnered critical support since it was announced this week, despite lingering doubts about whether the latest attempt to calm down the Syrian civil war can succeed.

Saudi Arabia, a key backer of the Syrian opposition and rebel factions, has gotten behind the agreement, with King Salman speaking with Russian President Vladimir Putin by telephone to discuss the details.

Putin also spoke with his ally, Syrian President Bashar al-Assad, who confirmed that his forces would take part in the deal.

Doubts remain about the sincerity of those signing up for the ceasefire. The exclusion of al-Qaeda, which fights alongside rebels on the frontlines and makes distinction between the groups difficult, has also raised concerns about whether the deal can hold.

Putin and al-Assad emphasized the necessity of continuing an "uncompromised fight" against UN-designated terrorist groups, such as Islamic State and al-Nusra Front, the Kremlin said after the phone call.

Russia in September started airstrikes in Syria to back al-Assad, a year after the United States began an aerial campaign against Islamic State militants.

The Russian intervention has bolstered al-Assad's government and put the rebels at a disadvantage, all while ensuring that Moscow is increasingly dominating the course of events Syria, now entering its fifth year of civil war.

Turkey has been pushed into a corner by recent events. Steven A. Cook, an analyst writing for the Council on Foreign Relations, says Turkey wants the US to choose between Ankara and the YPG.

In Syria, the US needs the Kurds to fight Islamic State but Turkey is a NATO-member and has opened its airbase to the US.

"It is a bind for American officials," Cook said. Since neither option serves the US interest, "the administration has split the difference," he added.

Turkey is fighting the banned Kurdistan Workers' Party (PKK), recognized by the US as a terrorist group, on its own soil. Links between the PKK and the YPG worry Ankara.

Rebels backed by Turkey have also been striking at YPG held areas in northern Syria, Kurdish sources said.

The Syrian Observatory for Human Rights, a monitoring group, said the intensity of Russian airstrikes has declined in recent days since the ceasefire deal emerged and the attacks seem more focused on Islamic State militants.

Russia had previously been accused of largely attacking rebel factions fighting al-Assad while ignoring Islamic State.

Parties to the civil war have until Friday at noon Damascus time (1000 GMT) to confirm if they are participating. It remains unclear if key hardline Islamic rebel groups will join.

Also on Wednesday, the UN completed its first humanitarian aid airdrop to Deir ez-Zor, a Syrian city besieged by Islamic State extremists, UN humanitarian chief Stephen O'Brien told the UN Security Council.

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