From regime change to retaliation, the BBC's Frank Gardner outlines possible outcomes of US strikes on Iran.

By Parisa Hafezi DUBAI, Jan 29 (Reuters) - Plainclothes Iranian security forces have rounded up thousands of people in a campaign of mass arrests and intimidation to deter further protests after crushing the bloodiest unrest since the 1979 Islamic Revolution,

Maritime intelligence warns of heightened risks for U.S.-flagged vessels in Persian Gulf as Iran threatens "immediate, all-out" response to any American military action.

US forces will conduct a multi-day air exercise in the Middle East as Washington bolsters its military presence in the region amid tensions with Iran.

The president advised Iran make a deal or "the next attack will be far worse."

He says a "massive Armada" is "moving quickly, with great power, enthusiasm, and purpose" towards Iran.

Iran's bloody crackdown on nationwide protests killed at least 6,126 people while many others still are feared dead, activists said Tuesday, as a U.S. aircraft carrier group arrived in the Mideast to lead any American military response to the crisis.

Oil prices climbed, with Brent crude back above $70 a barrel on growing concerns about potential U.S. military action against Iran.