Iran will treat any attack "as an all-out war against us," a senior Iranian official said on Friday, ahead of the arrival of a US military aircraft carrier strike group and other assets in the Middle East in the coming days.

"This military build-up — we hope it is not intended for real confrontation — but our military is ready for the worst-case scenario. This is why everything is on high alert in Iran," said the senior Iranian official, speaking on condition of anonymity.

"This time we will treat any attack — limited, unlimited, surgical, kinetic, whatever they call it — as an all-out war against us, and we will respond in the hardest way possible to settle this," the official said.

US President Donald Trump said on Thursday that the United States had an "armada" heading towards Iran but hoped he would not have to use it, as he renewed warnings to Tehran against killing protesters or restarting its nuclear programme.

"If the Americans violate Iran's sovereignty and territorial integrity, we will respond," said the Iranian official. He declined to specify what an Iranian response might look like.

"A country under constant military threat from the United States has no option but to ensure that everything at its disposal can be used to push back and, if possible, restore balance against anyone who dares to attack Iran," the official said.

The US military has in the past periodically sent increased forces to the Middle East at times of heightened tensions, moves that were often defensive. However, the US military staged a major build-up last year ahead of its June strikes against Iran's nuclear programme.