Tokyo, Feb 4 (IANS): US Defence Secretary James Mattis on Saturday described Iran as the world's greater sponsor of terrorism, hours after the Trump administration announced fresh sanctions on the country following a recent missile test.

"As far as Iran goes, this is the single biggest state sponsor of terrorism in the world," Mattis told reporters in Tokyo, before adding that he saw no need to increase troop numbers in the Middle East in response.

"I do not see any need to increase the number of forces we have in the Middle East at this time," Mattis said as he wrapped up his visits to Japan and South Korea, his first foreign trip as defence secretary.

Michael T. Flynn, US President Donald Trump's national security adviser, said this week that the US was putting Iran "on notice" because of its January 29 missile test and support for Houthi rebels in Yemen, whom the US has accused of threatening American vessels in the Red Sea and attacking a Saudi Navy patrol boat, the New York Times reported.

The Trump administration imposed economic sanctions on Friday against 25 Iranians and companies that it said were connected with Iran's missile programme and the country's Islamic Revolutionary Guard Corps. 

Mattis defended the decision to put a spotlight on Iran's behaviour, saying that it was important to make Iran recognise that "it is getting the attention of a lot of people", according to the report.

He also said that the US did not need to deploy additional military resources to signal its concern. "Right now, I do not think that is necessary," he said.