The conflict has closed the Strait of Hormuz, a key oil route, affecting global markets.
The crews of all three merchant vessels seized by the U.S. military over the past week are still in U.S. custody.
In a social media post Thursday morning, he said the military is intensifying its mine clearing efforts in the critical waterway.
President Trump says China has agreed not to provide weapons to Iran as reports circulate that Beijing has considered transferring arms.
Iran responded with threats on all ports in the Persian Gulf and the Gulf of Oman, taking aim at U.S.-allied countries.
The blockade, announced by U.S. Central Command, will affect vessels entering or leaving Iranian ports.
Israel has attacked a major petrochemical plant in Iran's South Pars natural gas field and killed a top Revolutionary Guard commander.
The search is on for one missing U.S. service member while another was rescued after two U.S. warplanes went down in separate incidents.
Two U.S. military planes were shot down in separate incidents on Friday, escalating tensions in the ongoing conflict.
It would mark the first time the U.S. has lost aircraft in Iranian territory and marks a dramatic escalation in the war since it began five weeks ago.