US President Barack Obama has written to Iran's Supreme Leader Ayatollah Ali Khamenei to express a "shared interest" in fighting the Islamic State extremist group in Iraq and Syria, a media report said here Thursday.The letter was sent in the middle of last month, in which the president tied potential cooperation on fighting extremists to a comprehensive deal over Iran's controversial nuclear programme, Xinhua quoted a Wall Stree Journal report as saying.In the letter, Obama stressed that "any cooperation on Islamic State was largely contingent on Iran reaching a comprehensive agreement with global powers on the future of Tehran's nuclear programme by Nov 24 diplomatic deadline", the report added. Foreign ministers of the US, the European Union and Iran are scheduled to meet in Muscat, Oman's capital, Nov 9-10, as Tehran and the P5+1 group of Britain, China, France, Russia, the US plus Germany are set to resume talks Nov 18 in the Austrian capital of Vienna to push for a final deal by the cut-off date.