The US said Thursday it has worked with Turkey to impose sanctions on four people and two firms that it says provided financial support to the Islamic State group. The announcement signalled counterterrorism cooperation between the two countries at a time of tensions over efforts to fight Islamic State. Turkish President Tayyip Erdogan has threatened an offensive into Syria against Kurdish militants he has blamed for a deadly Nov. 13 bombing in Istanbul. That has alarmed US officials. Kurdish groups have been allied with the US in the fight against Islamic State group and have warned that a Tu…