"As far as Iran goes, this is the single biggest state sponsor of terrorism in the world," Mattis said Saturday at a press conference in Tokyo, Japan, but added that Washington had no plans to increase troop numbers in the Middle East in response.
The Pentagon chief said ignoring Iran’s actions like former President Barack Obama would not work, without citing Obama or his policies.
"It does no good to ignore it. It does no good to dismiss it and at the same time I don't see any need to increase the number of forces we have in the Middle East at this time," he said.
The rhetoric was in line with those of Trump, who has pledged a more aggressive policy towards Tehran.
The comments also came less than a week after Mattis spoke by phone with his Saudi counterpart, Deputy Crown Prince Mohammad bin Salman Al Saud.
The Pentagon chief underlined the importance of the US-Saudi strategic ties, particularly with regard to security challenges in the Middle East.
The two officials further discussed what they claimed to be Iran’s "suspicious activities and interventions” in the region, the state-run Saudi Press Agency (SPA) said in a report.
This combo image shows US Defense Secretary James Mattis (L) and Saudi Deputy Crown Prince Mohammad bin Salman Al Saud, who also serves as the kingdom’s defense minister.
This is while Riyadh stands accused of supporting terrorist groups fighting the Syrian government since 2011, when the conflict broke out in the Arab country.
Washington and Riyadh have both been sidelined in efforts led by Iran, Russia and Turkey aimed at bringing an end to the crisis in Syria.
The Trump administration announced new sanctions against Iran on Friday, just two days after the White House put Iran "on notice" following a ballistic missile test.
The 13 individuals and 12 entities subject to new restrictions cannot access the US financial system or deal with American companies.
Those individuals and entities, which are based in Iran, the United Arab Emirates, Lebanon and China, are also subject to secondary sanctions, meaning foreign companies and individuals are prohibited from dealing with them or risk being blacklisted by Washington.
However, the fresh sanctions do not affect the lifting of broader US and international sanctions that took place under a landmark nuclear agreement.
Iranian officials have dismissed the US threats as futile, stressing that the country will keep boosting its defensive capabilities.
Tehran maintains that its missile launches fit into its conventional defense doctrine and has nothing to do with the nuclear agreement signed between Iran and the six world powers in 2015.