Turkish President Recep Tayyip Erdogan has slammed the US' support for Kurdish YPG fighters in northern Syria, warning that Washington's move to fund the group will impact Ankara's future decisions towards its NATO ally.

"The US decision to give financial support to the YPG ... will surely affect the decisions Turkey will take," Erdogan said on Tuesday.

"It will be better for them not to stand with the terrorists they support today. I am calling on the people of the United States - this money is coming out of the budget of the US, it is coming out of people's pockets," Erdogan said.

His comments were a response to the Pentagon's new defence budget that allocates $550m to military activities in Syria.

The US defence department requested $300m for Syrian "train and equip activities" and $250m for "border security requirements related to the counter ISIS mission," according to a copy of the budget.

Turkey last month started a military incursion, dubbed "Operation Olive Branch", into the Kurdish-held Afrin region in northwestern Syria to sweep Kurdish YPG fighters from its border there.

Top Turkish officials have threatened to extend the incursion to the Syrian town of Manbij, which is under the control of YPG-led Syrian Democratic Forces (SDF), and warned US troops stationed there not to get in the way.

The US has reiterated that it has no plans to withdraw its forces from Manbij.

Paul Funk, the commander of US forces in Syria and Iraq, made a recent visit to Manbij and said that the US and its partners in Syria would hit back if attacked.

"You hit us, we will respond aggressively. We will defend ourselves," Funk said.

Erdogan took aim at that, saying: "It is obvious that those, who say they will 'give a sharp response' if they were hit, have not been hit by the Ottoman slap."

The "Ottoman slap" was a potentially fatal martial arts technique used by an elite infantry of the Ottoman Empire in the 17th century.

It was used when a soldier lost or dropped his weapon in battle and was supposedly devastating enough to cause a concussion or even break the enemy's neck.

Tit-for-tat

The US believes that the Turkish move into northern Syria undermines the larger fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

US Secretary of State Rex Tillerson called on allies to unite in the fight against ISIL and criticised Turkey's operation in Afrin.

"The end of major combat operations does not mean we have achieved the enduring defeat of ISIS," Tillerson said on Tuesday in meeting in Kuwait.

"[Turkey's operation] has detracted from our fight to defeat Islamic State in eastern Syria … Forces have diverted from there to Afrin," he said, referring to ISIL.

Erdogan, however, said that the US could no longer use fighting ISIL as an excuse, as the group - according to the president - has been largely defeated in Syria.

"From now on, nobody has the right to use the Daesh as an excuse. The theatre of ISIL has ended," he said.

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The US has long supported the SDF as a "reliable ally" in its fight against ISIL, despite strong objections by Ankara, which sees the YPG, leading the coalition, as the Syrian branch of the outlawed Kurdistan Workers' Party (PKK).

About 40,000 people have been killed in Turkey since the PKK launched its fight against the Turkish state in the 1980s, seeking more autonomy for the Kurds.

Tillerson is expected to visit Turkey on Thursday.

Tillerson's planned visit to Turkey shows just how "serious this matter is," Department of State Spokesperson Heather Nauert said on Tuesday, referring to the stark differences in Turkish and US interests in Syria during a daily briefing.

"This is one of the areas of deep, deep concern on the part of the administration," Nauert told reporters in Washington.

"We certainly don't want to see violence to further escalate."