Turkey has denied targeting a US base in northern Syria after the Pentagon said its troops had come under artillery fire.

"There was no shot fired whatsoever on the US observation post," Defence Minister Hulusi Akar said in a statement to state news agency Anadolu.

He said Turkey had returned fire after Kurdish People's Protection Units (YPG) shelled a Turkish border police station from hills located 1km away from the US observation post in Syria.

The Pentagon said an explosion occurred "within a few hundred metres" of a US position near the Syrian town of Kobani, and warned that the US was prepared to meet aggression with "immediate defensive action".

Mr Akar said: "All necessary precautions were taken so as not to damage the US post."

He said the Turkish forces had stopped firing "as a precaution" after the Americans contacted them.

"Anyway, the necessary coordinations are being conducted between our command centres and the Americans," Mr Akar added.

US troops pulled back from positions along the Turkey-Syria border last week ahead of a Turkish operation against Kurdish militants in Syria.

The YPG was a close ally of the US in its fight against the so-called Islamic State group but is seen by Turkey as a "terrorist" off-shoot of Kurdish insurgents in Turkey.

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US Treasury Secretary Steven Mnuchin said that US President Donald Trump had authorised - but not yet activated - "very significant new sanctions" to dissuade Turkey from further offensive military action.

The United Nations has said around 100,000 people have been forced from their homes in northeastern Syria since the Turkish military offensive began last Wednesday. 

The death toll among Syrian Kurdish-led fighters battling the offensive has risen to 74, most of whom have been killed in the Tel Abyad area, the Syrian Observatory for Human Rights said.

Observatory Director Rami Abdulrahman also said 49 fighters with Turkish-backed Syrian rebel groups had been killed since the assault began.

The death toll among civilians in Syria had climbed to 20 after two people died in the city of Qamishli, he said.

Most of the civilian deaths were also in Tel Abyad, a border town that is one of the focal points of the operation.

Meanwhile, the British Prime Minister, Boris Johnson, voiced his "grave concern" that the action could worsen the humanitarian situation in the region and undermine the fight against so-called Islamic State (IS).

He called on him to enter into dialogue with a view to reaching agreement on a ceasefire.

"He expressed the UK's grave concern about Turkey's military operation in northern Syria which he said could further worsen the humanitarian situation there and undermine the progress made against Daesh (IS)," a government spokesman said.

Explained: Syria's War

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