Turkish President Recep Tayyip Erdogan has vowed Ankara would continue to purchase oil and natural gas from Iran despite US sanctions. "It is impossible for us to cancel relations with Iran with regards to oil and natural gas. We will continue to buy our natural gas from there," Erdogan told Turkish reporters before leaving New York where he was attending the UN General Assembly. Despite this vow, Erdogan admitted Turkey faced difficulty in purchasing oil since the private sector "pulled back because of US threats", NTV broadcaster reported. "But on this issue especially and many other issues, we will continue our relations with Iran," he promised, adding that Ankara still sought to increase trade volume with Tehran. The United States reimposed sanctions on Iran after pulling out of the landmark 2015 nuclear deal, and says it aims to reduce Tehrans energy sales to zero. Erdogan previously criticized the sanctions, insisting that they achieved nothing. NTV also quoted the Turkish president as saying that preparations by Ankara and Washington on a safe zone for refugees in northeastern Syria are on schedule. “The schedule is moving along, all our preparations along the border are also complete." "Upon returning (to Turkey), we will hold evaluations … on what sort of steps to take and implement them ... because Turkey is not a country that can be stalled," he said. Turkey and the US started joint land and air patrols along part of Syrias border with Turkey. Ankara wants Washington to clear the Syrian Kurdish YPG from a 480-km-long border area, and Erdogan warned that Turkey would act unilaterally if the group was not removed.