A U.S. decision to continue funding the Syrian Kurdish YPG militia will affect Turkey's future moves, President Tayyip Erdogan said on Tuesday ahead of a visit this week by Secretary of State Rex Tillerson.

U.S. officials have said that Tillerson expects to have difficult conversations when he visits Turkey on Thursday and Friday, given that the NATO allies have starkly diverging interests in Syria.

Turkey has been enraged by U.S. support for the YPG, which Ankara sees as a terrorist organisation and an extension of the outlawed Kurdistan Workers Party that has been waging an insurgency on Turkish soil for over 30 years. Washington has backed the YPG in the fight against Islamic State in Syria.

"Our ally's decision to give financial support to the YPG... will surely affect the decisions we will take," Erdogan said in a speech to members of his AK Party in parliament.

Turkey last month launched a military incursion, dubbed "Operation Olive Branch", into the Kurdish-held Afrin region of Syria to sweep the YPG away from its southern border.

It has also threatened to press on to the Syrian town of Manbij, under the control of a YPG-led force, and warned U.S. troops stationed there not to get in the way.

"It is very clear that those who say 'we will respond aggressively if you hit us' have never experienced an Ottoman slap," Erdogan said in parliament.

That was an apparent reference to comments made by U.S. Lieutenant General Paul Funk during a visit to Manbij.

But Washington says it has no plans to withdraw its soldiers from Manbij and two U.S. commanders visited the town last week to reinforce that message.

The Pentagon has requested $300 million for Syrian "train and equip activities" and $250 million for border security requirements, according to a copy of the U.S. Department of Defence's 2019 budget.

While it did not specify how much of this, if any, was earmarked for YPG-led forces, Turkish media interpreted that to mean that the Pentagon had allocated $550 million to the YPG in 2019.

"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 United States, it is coming out of people's pockets," Erdogan said.