Turkish warplanes carried out deadly strikes on US-backed militias in northern Syria, including Syrian Kurdish fighters, a monitor said Thursday, action that is likely to raise tensions between the NATO allies.

The army, quoted by the official news agency Anadolu, said the raids hit 18 targets north of the battered Syrian city of Aleppo, areas recently recaptured by the People's Protection Units (YPG) from the Islamic State group.

It said the raids killed between 160 and 200 YPG militants.

The Syrian Observatory for Human Rights in Beirut, however, put the toll much lower at at least 11 fighters from the Syrian Democratic Forces (SDF)-- a US-backed alliance of Kurdish and Arab fighters -- killed and 24 wounded.

Tensions between the US and its NATO ally Turkey have heightened over Ankara's actions against the YPG, which Washington views as an effective force against IS in Syria.

Ankara has repeatedly said it will not allow a "terror corridor" on its southern border and wants to prevent the joining of the Kurdish "cantons" of Afrin and Kobane.

The strikes come on the eve of a visit to Turkey by US Defence Secretary Ashton Carter to discuss developments in the region including the Syrian conflict and the offensive to recapture Iraq's second city of Mosul which began this week.

Ankara considers the YPG and the Kurdish Democratic Union Party (PYD) as terror groups linked to the outlawed Kurdistan Workers' Party (PKK).

The PKK, proscribed as a terrorist organisation by the United States and the European Union, has waged a bloody insurgency against the Turkish state since 1984.

In a statement, the Kurdish Rojava region in Syria condemned Ankara's actions as "blatant aggression", calling for the United Nations, Moscow and Washington to "put direct pressure on Turkey to stop its attacks".

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President Recep Tayyip Erdogan has previously said Turkey and Washington had been discussing a joint operation to take territory back from IS but has warned the US not to launch any offensive with the YPG.

Michael Stephens, senior research fellow at the London-based Royal United Services Institute said Turkey's actions were "a message and a warning to the YPG 'understand something, you won't join these two cantons up. There are going to be consequences'".

"If there is a signal that that is what is happening, the Turks will fight it and they will use the premise of national security to do that," he told AFP.

The strikes were part of Turkey's military operation in northern Syria launched on August 24. Ankara has sent in tanks and has been striking jihadist targets while supporting Syrian opposition fighters in their battle to retake IS-controlled territory.

The goal of the operation was to remove IS from the Turkish border -- which last month Ankara said it achieved -- while also aiming to halt the westward advance of the YPG.

Turkish media quoted the army saying shells fired on Thursday from YPG-controlled Afrin region hit uninhabited land in Hatay, southern Turkey, which the military responded to with howitzer fire.

Erdogan said Wednesday that Turkey would not "wait for terrorist organisations to come and attack us".

"Instead of dealing with the flies, we will drain the swamp," he said in a speech in Ankara.

Erdogan has previously said he wants to create a 5,000-square-kilometre (1,900 square-mile) safe zone in Syria by pushing further south in the operation dubbed "Euphrates Shield".