Trump is due to meet on Tuesday with leaders of the six Gulf Arab states: Bahrain, Kuwait, Qatar, Oman, Saudi Arabia and the United Arab Emirates.
All are keen to restore shipping through the Strait of Hormuz but wary of giving their longstanding rival Iran too much power. All apart from Oman also host huge US military bases that have come under repeated Iranian attack throughout the war.
The biggest of them, Saudi Arabia, has been drawn deeper into the war this month, with Houthi fighters aligned to Iran advancing in Yemen, routing forces of a Saudi-backed government there and threatening Saudi oil exports through the Red Sea.
There was some relief on oil markets on Tuesday, after sources told Reuters that Saudi Arabia had restarted operations at its East-West Pipeline and could resume exports within hours, albeit at reduced volume, from the Red Sea port of Yanbu. O/R
The pipeline, Saudi Arabia's main alternative route for exports to bypass wartime disruption in the Strait of Hormuz, was shut down after attacks this month that Riyadh blamed on Iranian allies in Iraq. O/R
The Houthis have routed Saudi-supported government forces along the Red Sea and the Bab el-Mandeb Strait at its mouth, and have attacked Saudi Arabia directly with missiles and drones. The fighters are now pushing to seize the strategic heights of the Kahboub Mountains that overlook the Red Sea coast.
The US, which bombed the Houthis for two months last year, has so far rebuffed repeated pleas from Saudi Arabia's Crown Prince Mohammed bin Salman to rejoin the fight. The New York Times reported that Trump called off US strikes against the Houthis on Sunday at the last minute, after troops had already begun loading bombs onto planes.
Asked why Washington had so far held back from striking the Houthis, Rubio said: "Ultimately, if it comes down to it, the United States will always stand with Saudi Arabia and support them because we have an agreement with them to do so.
"That said, that is a very dangerous and complex conflict that pre-exists... We have a history with the Houthis as well, and that's something we don't want to see impacted either."