Turkish armed forces are besieging the Syrian Kurdish-controlled district of Afrin in northern Syria, Turkey's president has said.

In a televised statement on Friday, Recep Tayyip Erdogan said the Turkey-backed Free Syrian Army fighters and Turkish troops were 6km from the centre of Afrin.

"Now the centre of Afrin is surrounded and our entry is imminent," he said.

"We are removing the last remaining obstacles standing before our besieging of Afrin city centre."

However, a spokesperson for the Kurdish People's Protection Units (YPG) troops contradicted the claims.

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"The forces of Erdogan's Turkish army ... are 10 to 15km away from it [Afrin]," Nouri Mahmoud said.

"Today, there were also clashes around Bulbul, where they announced around 30 days ago that they captured, occupied it."

The assault on Afrin is part of Operation Olive Branch, a Turkish military offensive to remove Kurdish fighters in northwestern Syria.

In his speech, Erdogan said the military operation would not end with the seizure of Afrin.

"We are in Afrin today; we will be in Manbij tomorrow. The next day we will ensure that the east of Euphrates will be cleared of terrorists up to the Iraqi border," he said.

Turkey launched the operation in northern Syria on January 20 against the US-backed YPG in its border stronghold of Afrin in northwestern Syria. Turkey sees the YPG as a "terrorist" group.

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The YPG, the armed wing of the Kurdish Democratic Union (PYD) Party in Syria, is considered a "terrorist group" by Turkey.

Turkey says they have ties to the outlawed Kurdistan Workers' Party (PKK), active in Turkey.

The PKK has waged a decades-long armed fight against the Turkish state that has killed tens of thousands of people.

Relations between NATO allies US and Turkey have been strained over a number of issues, notably the US arming of the YPG.

Turkey has repeatedly called on the US to stop working with the YPG.