“Turkey cannot accept any kind of cooperation with terror organizations that have declared war against Turkey,” Prime Minister Ahmet Davutoglu said. (Murad Sezer/Reuters)

Turkey has summoned the U.S. ambassador to register its displeasure over reports of growing ties between the United States and Kurdish militants in Syria, the Turkish prime minister said Wednesday.

The Turkish objection comes amid calls from some quarters for closer U.S. cooperation with the Kurds to oust the Islamic State militant group from its strongholds in war-torn Syria.

Prime Minister Ahmet Davutoglu said Wednesday that armed Kurds in Syria could cooperate with Kurdish militants battling security forces inside Turkey.

Davutoglu said at a televised news conference that he also warned Russia against backing the Kurdish People’s Protection Units, known as the YPG, in Syria. Russia began launching airstrikes against Syrian rebels on Sept. 30 in a bid to strengthen President Bashar al-Assad, its embattled ally. Although the YPG, a secular force, has cooperated with some Syrian rebels, it has also maintained relations with Assad’s government.

“Turkey cannot accept any kind of cooperation with terror organizations that have declared war against Turkey,” Davutoglu said.

No one can guarantee that arms given to the YPG “won’t tomorrow fall into the hands of the PKK and be used against Turkey,” he said, according to the Associated Press. The PKK, or the Kurdistan Workers’ Party, is deemed a terrorist organization by Turkey and the United States.

Tensions between Ankara and Washington have been exacerbated by reports that the U.S. military air-dropped arms and ammunition earlier this week to rebel groups preparing to fight the Islamic State in northern Syria. A Kurdish official in Syria said arms were also sent to the YPG, according to the Associated Press.

The United States did not confirm the reports that it had supplied weapons to Kurdish fighters, who have routed the Islamic State from key areas. But there is a growing push, including from some U.S. officials, for the U.S. military to cooperate more closely with the YPG. Concerns about ties to extremists have made the United States wary of closer collaboration with Syria’s rebels, many of whom are Islamist.

In July, the United States and Turkey announced a deal that would include support for Syria’s moderate rebels to create an Islamic State-free buffer zone along the Turkish border. But as Russia continues to strike rebel targets in Syria, the United States scrapped a training program for Syrian fighters and said it would directly arm groups battling the Islamic State.

“The U.S. is saying, ‘Okay, we want to focus on the YPG. They are better fighters and better organized. Maybe in the future they will get more sophisticated weapons,’ ” said Serkan Demirtas, the Ankara bureau chief for Turkey’s Hurriyet Daily News. “But Turkey is afraid they [the YPG] will turn the weapons back against Turkey.”

In July, Defense Secretary Ashton B. Carter acknowledged that the YPG is a “capable” and “effective” force.

But although U.S. officials see the Kurds as more palatable allies, Turkey, a NATO member and key ally of Washington, is worried that the success of Syria’s Kurds would inspire fellow Kurds in Turkey to boost their fight for an independent state.

“There is an enormous level of disagreement between Turkey and the United States on this,” Demirtas said of working with the YPG to fight the Islamic State. “This relationship [between Turkey and the United States] is not ideal. There are fault lines in this alliance.”

Amid Syria’s civil war, the Kurds have carved out a de facto autonomous region. The conflict has divided the country into a number of fiefdoms controlled by rebel groups, the Islamic State and the government. Turkey has vowed to prevent the Kurds from establishing a Syrian-Kurdish state on its border.

Amnesty International this week accused the YPG of potential war crimes, citing allegations that YPG fighters razed the homes of Arab Syrians and other ethnic groups in areas now under their control. The London-based rights group said the YPG is waging an ethnic-cleansing campaign in parts of Syria.

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