Secretary of State Marco Rubio said on Wednesday that "no one knows" who would replace Iran's supreme leader Ayatollah Ali Khamenei if the regime in Tehran was toppled, and that the US reserved the option for "pre-emptive defence".
Mr Rubio was speaking before the Senate foreign relations committee during a hearing on US action in Venezuela this month that led to the capture of the country's leader, Nicolas Maduro.
He told the committee that a replacement for Mr Khamenei was "an open question. I mean, no one knows who would take over".
Widespread protests that began over economic grievances rocked Iran for weeks, but have been mostly suppressed by a brutal government crackdown. Mr Rubio said the protests have left Iran "weaker than ever".
"I know the President [Donald Trump] is very concerned about the what's happening to the demonstrators," Mr Rubio said. "And of course, there's some estimates that as many as 30,000 Iranian demonstrators have been executed by the regime."
Senators questioned Mr Rubio over recent concerns that Mr Trump was considering another strike on Iran due to Tehran's response to the protests. The US attacked the country's nuclear sites in June.
Mr Trump repeated on Tuesday that an "armada" was on its way to the Middle East to bolster the US presence in the region.
Mr Rubio said that tens of thousands of American troops stationed in the region were within range of an attack by Iran, which has increased its threats
He said that Mr Trump has been presented with a range of possible actions but that he always reserves the "pre-emptive defensive option".
"We have to have enough force and power in the region, just on a baseline, to defend against that possibility [of an Iranian attack] at some point," Mr Rubio told senators.
He said the Venezuela-Iran connection was part of the reason the US moved to strike Caracas and capture Mr Maduro.
"Iran, their primary foothold in the region was Venezuela," Mr Rubio said. "And it involved more than just selling weapons … there's clear evidence now that they were providing Venezuelan passports [that] were false identities to people who operated on behalf of both Iran and Hezbollah."
He said that Iran was involved in shipping sanctioned Venezuelan oil. The US has taken control of the Venezuelan oil industry and has placed about $300 million in profits in an account – that Washington administers – in Qatar, allowing Caracas to use the funds for approved purposes.
Mr Rubio emphasised that the US was not at war in Venezuela, and that the goal was for no American military presence in the country.
"But that said, if an Iranian drone factory pops up and threatens our forces in the region, the President retains the option to eliminate that," he said.
