New Delhi, Jan. 28 -- Iran's bloody crackdown on nationwide protests has killed at least 6,221 people while many others still are feared dead, activists said Wednesday, as a U.S. aircraft carrier group arrived in the Middle East to lead any American military response to the crisis. Iran's currency, the rial, meanwhile fell to a record low of 1.5 million to $1.

The arrival of the USS Abraham Lincoln aircraft carrier and guided missile destroyers accompanying it provide the U.S. the ability to strike Iran, particularly as Gulf Arab states have signaled they want to stay out of any attack despite hosting American military personnel.

Two Iranian-backed militias in the Mideast have signaled their willingness to launch new attacks, likely try...