ISTANBUL (Reuters) - Turkish President Tayyip Erdogan said on Sunday countries which promised to get back weapons supplied to Kurdish YPG fighters in northern Syria were trying to trick Turkey and would eventually realize their mistake.

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Ankara was infuriated by a U.S. decision last month to arm the YPG, which Washington sees as a vital ally in the battle against Islamic State in its Syrian stronghold of Raqqa but which Turkey deems an extension of the outlawed Kurdish PKK.

The PKK, designated a terrorist group by the United States, the European Union and Turkey, has been waging an insurgency in southeastern Turkey since the 1980s.

Turkish defense ministry sources said on Thursday the United States had pledged that weapons provided to the YPG would be taken back once Islamic State was defeated.

However Erdogan, addressing members of his ruling AK Party in a speech marking the start of the Islamic Eid holiday, appeared to dismiss those assurances, saying Turkey’s friends and allies were cooperating with terrorists.

“The ones who think they are tricking Turkey by saying they are going to get back the weapons that are being given to this terrorist organization will realize that they are making a mistake eventually,” he said.

“But it will be too late for them,” he added, saying that if violence spilled over Syria’s border into Turkey, Ankara would hold to account anyone who supplied arms to the YPG.

“We will make the real owners of those weapons... pay for any bullet that will be fired to our country, for every drop of blood that will be shed,” he said.

U.S. President Donald Trump decided to arm the YPG fighters, who form a main part of the U.S.-backed force fighting Islamic State inside Raqqa, despite protests from NATO ally Ankara and a direct appeal from Erdogan at a White House meeting last month.

Erdogan said the decision contravened the military alliance’s framework of cooperation.

Faced with turmoil across its southern border, Turkey last year sent troops into Syria to support Syrian rebels fighting both Islamic State and Kurdish forces who control a large part of Syria’s northern border region.

“I want all the world to know that in northern Syria, on our border, we are never going to allow a terrorist state to be established,” Erdogan said.