Donald Trump has asked the Treasury department to “substantially” toughen economic sanctions on Iran, as the US weighs its response to a strike against Saudi Arabian oil facilities that knocked out more than half the kingdom’s production.

The US president made the announcement in a tweet yesterday, shortly after appointing Robert O’Brien, the state department’s envoy for hostage affairs, to be his next national security adviser. Mr O’Brien, a lawyer, replaces John -Bolton, who abruptly left the White House team this month.

Mr Trump’s demand for more aggressive economic sanctions suggests he is taking a more cautious approach to the crisis triggered by the attack on the Saudi oil facility, which Mike Pompeo, his secretary of state, said was an “act of war” against the kingdom.