US President Donald Trump said Sunday he was considering potential military action against Iran, amid mounting reports of deadly crackdowns against the country's mass anti-government protests.

"They're starting to, it looks like," Trump said, when asked by reporters aboard Air Force One if Iran had crossed his previously stated red line of protesters being killed.

"We're looking at it very seriously. The military is looking at it, and we're looking at some very strong options. We'll make a determination," he said.