Rubio made his comments in a meeting of Southeast Asian foreign ministers a day after three oil tankers carrying Saudi crude to Asia reversed course in the Red Sea, apparently in response to threats from Yemen's Iran-aligned Houthis.
The Houthis, who control the coast at the mouth of the Red Sea, announced a naval blockade on Saudi Arabia on Monday, opening a potential new front in the war which has killed thousands of people across the Gulf since it began on February 28 with US and Israeli attacks on Iran.
With Iran already threatening shipping through the Strait of Hormuz leading out of the Gulf, the Red Sea has served as the main alternate route out for millions of barrels of Saudi oil per day.
A senior Iranian official told Reuters on Monday that Tehran had received a proposal from mediators for a 10-day ceasefire in efforts to salvage the interim ceasefire agreement signed by the US and Iran in June, which replaced an earlier April ceasefire.
In another sign that diplomacy remains alive, Iranian Interior Minister Eskandar Momeni has visited mediator Pakistan and asked Islamabad to continue its efforts.
Rubio said Washington was "always committed to diplomacy" but doubted whether Tehran was equally committed to negotiations.
"The problem we're having right now is that they're not serious about talks. If they're serious, we're serious. If they're not, then we will do what's necessary to protect our interests, and also the interests of our allies," Rubio said in Manila.
He stressed that Iran could not be allowed to control the Strait of Hormuz, arguing it would create a dangerous precedent for the world including Southeast Asian countries, many of which have territorial disputes in the South China Sea with China.
With no diplomatic breakthrough in sight, the US military bombed targets across Iran for an 11th straight night on Tuesday.
Tehran residents reported hearing explosions in the early hours of Wednesday as Iran activated its air defences over the capital, Iran's semi-official Fars news agency said.
Three locations in Iran's Bushehr Province, home to Iran's only nuclear power plant, were hit by US attacks early on Wednesday, an official said, including an electricity post close to the plant.
Iran's army said it targeted US military facilities in Kuwait, Jordan and Bahrain with drones early on Wednesday.
US President Donald Trump confirmed that 18 US service members had been killed in the war, including four in Iranian attacks on US military bases in Jordan and Iraq over the last few days.
In a letter to shippers, the Houthis on Tuesday threatened to attack any ships that load or discharge Saudi oil.
Trump said the Houthis had not yet shut the Bab el-Mandeb, the "Gate of Tears" strait leading into the Red Sea, and threatened to act against them if they did.
"So far it hasn't happened," Trump said.
"If something like that happens, we take care of it."
Three oil tankers loaded with Saudi crude for China and India made U-turns in the Red Sea on Tuesday, heading towards the Suez Canal rather than braving the Yemeni coast at the sea's mouth.
On Tuesday, Trump renewed his threats to again attack Iran's nuclear facilities at Natanz "pretty soon", which he said in June 2025 had been "totally obliterated" after the US military bombed the facility, buried in a mountain range, that month.
Iran promised it would retaliate.
with Reuters, PA and DPA
