Turkey’s president has said the country will launch a military assault on a Kurdish enclave in northern Syria “in the coming days” and urged the US to support its efforts.

Recep Tayyip Erdoğan said the operation against the Afrin enclave aimed to “purge terror” from his country’s southern border.

Afrin is controlled by a Syrian Kurdish militia known as the YPG. Turkey considers the YPG to be a terrorist group linked to the outlawed Kurdistan Workers’ party (PKK) that has waged a bloody insurgency within its borders.

A YPG spokesman in Afrin said there had been clashes on Sunday between his unit and Turkish troops near the border. Rojhat Roj said shelling of areas in Afrin district, in Aleppo province, had killed one YPG fighter and injured two civilians.

Turkey and its western allies, including the US, consider the PKK a terrorist organisation. But the US has been arming some of Syria’s Kurds to help defeat Islamic State in Syria – a sore point in already tense US-Turkish relations.

The Turkish president said that “despite it all” he wanted to work with the US in the region and hoped it would not side with the YPG during the Afrin operation.

“We expect [the US] to support Turkey in its legitimate efforts” to combat terror, Erdoğan said.

Separately, Erdoğan’s spokesman described as “worrying” reports that the US-led coalition fighting Isis would establish a 30,000-strong border security force in Syria involving the Kurdish militia.

In December the Associated Press reported that the US was developing an expanded training programme for Kurdish and Arab border guards in Syria to prevent the resurgence of Isis.

Ibrahim Kalin, Erdoğan’s spokesman, said the US was taking steps to legitimise and solidify the YPG. “It’s absolutely not possible to accept this,” Kalin said, and he repeated that Turkey would defend itself.

Erdoğan said the Afrin operation would be an extension of Turkey’s 2016 incursion into northern Syria, which aimed to combat Isis and stem the advance of US-backed Kurdish forces. Turkish troops are stationed in rebel-held territory on both sides of Afrin.

Roj, the YPG spokesman, said the Kurdish militia would fight to “defend our gains, our territories.” Hediye Yusuf, a senior Kurdish official, wrote on Twitter that the Turkish operation against Afrin was a “violation” of the Syrian people and undermined international efforts to reach a political solution in Syria.

The Turkey-PKK conflict has killed an estimated 40,000 people since 1984, including more than 3,300 state security forces, militants and civilians since the resumption of hostilities in July 2015.