US has called Iran the world's biggest state sponsor of terrorism, amid rising tensions between the two nations. Defence Secretary James Mattis' comments came a day after the Trump administration imposed new sanctions against Iran in response to a ballistic missile test. US officials said the sanctions, targeting 13 people and 12 companies, were in response to last Sunday's missile test and Iran's continued support for terrorism.They are the first Iran sanctions of Donald Trump's new presidency, who has signalled a tougher stance on Iran.