ISTANBUL/WASHINGTON: Turkish forces stepped up their bombardment around a town in northeast Syria on Saturday, the fourth day of an offensive against a Kurdish militia, after US troops in the region came under artillery fire from Turkish positions. The United States has ramped up its efforts to persuade Ankara to halt the incursion against the US-backed Kurdish forces, saying Ankara was causing “great harm” to ties and could face sanctions. Turkey opened its offensive after US President Donald Trump spoke by phone on Sunday with Turkey’s President Tayyip Erdogan and withdrew US troops who had been fighting alongside Kurdish forces. On Friday evening, Erdogan dismissed mounting international criticism of the operation and said Turkey “will not stop it, no matter what anyone says.” On the frontlines, thick plumes of smoke rose around Syria’s Ras Al-Ain, one of two border towns targeted in the offensive, as Turkish artillery targeted the area on Saturday, said a Reuters reporter across the frontier in the Turkish town of Ceylanpinar. Intense gunfire also resounded from within the town of Ras al-Ain itself, while warplanes could be heard flying overhead, he said. It was quieter at Tel Abyad, the operation’s other main target some 120 km to the west, with only occasional shell fire, heard in the area, another Reuters reporter said. Earlier, 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.” The US military confirmed an explosion around 9.00 pm (1800 GMT) within a few hundred meters 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 US forces have not withdrawn from Kobani,” Navy Captain Brook DeWalt said in a statement. 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 Daesh 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.”