Donald Trump has faced a firestorm of criticism for appearing to greenlight Turkey's offensive into Syria.

US troops near the northern Syrian border came under artillery fire from Turkish positions on Friday, a Pentagon spokesman said, warning that the US was prepared to meet aggression with "immediate defensive action."

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The US military confirmed an explosion around 9.00pm (1800 GMT) within a few hundred metres of its post near the town of Kobani, in an area "known by the Turks to have US forces present."

"All US troops are accounted for with no injuries. US forces have not withdrawn from Kobani," Navy captain Brook DeWalt said in a statement.

Steven Mnuchin takes a question from a reporter after announcing the threat of sanctions on Turkey. AP

US President Donald Trump has faced a firestorm of criticism for appearing to greenlight Turkey's offensive into northeastern Syria, which began after Trump ordered US troops to pull back from the border.

Turkey is targeting the Kurdish-led Syrian Democratic Forces (SDF), a key US ally in the five-year battle to crush the Islamic State group. The SDF lost 11,000 fighters in the US-led campaign.

Treasury Secretary Steven Mnuchin said earlier Friday Trump had authorized — but not yet activated — "very significant new sanctions" to dissuade Turkey from further offensive military action.

"The United States remains opposed to the Turkish military move into Syria and especially objects to Turkish operations outside the security mechanism zone and in areas where the Turks know US forces are present," DeWalt added.

"The US demands that Turkey avoid actions that could result in immediate defensive action."

Agence France-Presse