ISTANBUL/BEIRUT (Reuters) - Turkish air strikes pounded a group of Kurdish fighters allied to a U.S.-backed militia in northern Syria overnight, highlighting the conflicting agendas of NATO members Ankara and Washington in an increasingly complex battlefield.

A man walks past damaged buildings in the rebel held besieged al-Sukkari neighbourhood of Aleppo, Syria October 19, 2016. REUTERS/Abdalrhman Ismail

The jets targeted positions of the Kurdish-led Syrian Democratic Forces (SDF) in three villages, northeast of the city of Aleppo, that the SDF had captured from Islamic State, the Syrian Observatory for Human Rights said late on Wednesday.

The Turkish military confirmed its warplanes had carried out 26 strikes on areas recently taken by the Kurdish YPG militia, the strongest force in the SDF, and that it had killed between 160 and 200 combatants.

The British-based Observatory monitoring group reported a much lower toll of at least 14 dead and dozens wounded. Officials of the Kurdish-led administration that controls much of northeastern Syria said dozens had been killed.

Complicating matters further, Syria’s military called the strikes by Turkey an act of “blatant aggression” and said it would bring down any Turkish war planes entering Syrian air space.

A senior U.S. defence official said the groups struck by Turkish jets were not themselves U.S.-backed but were “close to and friendly with” the fighters Washington is working with.

Asked in the light of the air strikes whether he was concerned the U.S. alliance with Turkey was unravelling, Defense Secretary Ash Carter told a news conference at the Pentagon:

“With respect to Turkey, our partnership is very strong in the counter-ISIL campaign.” ISIL is an acronym for Islamic State. “We’re working with the Turks now very successfully to help them secure their border area,” added Carter, who was due to visit Turkey on Friday.

The United States has backed the Kurdish-led forces in their fight against Islamic State, infuriating Ankara, which sees the YPG as an extension of Kurdish PKK militants who have waged a three-decade insurgency in southeastern Turkey.

Turkey fears the YPG will try to connect three de facto autonomous Kurdish cantons that have emerged during the five-year war to create a Kurdish-run enclave in northern Syria, stoking the separatist ambitions of Kurds on its own soil.

Five shells fired from the YPG-controlled Afrin region of Syria, west of where the air strikes hit, landed in empty land in Turkey’s Hatay province on Thursday, triggering retaliatory howitzer fire from Turkey, the Turkish military said.

The Turkish army bombarded villages near Afrin overnight, the pro-Kurdish Anha news agency said. Footage purportedly showed smoke billowing out from the Syrian side of the border. Anha said there were casualties from the shelling by what it described as at least 44 howitzers.

The Turkish army also said 21 PKK militants had been killed in operations in Hakkari province in Turkey’s southeast, where violence has flared since the PKK abandoned a ceasefire in 2015.

“WE WILL NOT WAIT”

The air strikes, the heaviest against the YPG since Turkey launched a military incursion into Syria two months ago, came hours after President Tayyip Erdogan warned that Ankara could act alone in rooting out its enemies abroad.

“From now on we will not wait for problems to come knocking on our door, we will not wait until the blade is against our bone and skin, we will not wait for terrorist organisations to come and attack us,” Erdogan said in a speech on Wednesday.

The Observatory named the bombed villages as al-Hasiya, Um al-Qura and Um Hosh. They lie around 30 km (19 miles) west of al-Bab, the last big town held by Islamic State militants in northwest Syria after a series of battlefield reverses.

Turkey, a major backer of the insurgency against President Bashar al-Assad, entered the Syrian conflict in August, using its armour and air power to help Free Syrian Army rebel groups take territory near the border held by Islamic State.

But its intervention also aimed to prevent the SDF from gaining more ground. The SDF has been moving eastwards towards al-Bab, which Turkish-backed rebel forces also want to capture from Islamic State.

The Turkish military said its air strikes had destroyed nine buildings, one armoured vehicle and four other vehicles that belonged to the YPG.