WASHINGTON (Reuters) -- Iran is probably the No. 1 challenge to the United States, Secretary of State Condoleezza Rice said on Thursday after Tehran vowed no compromise in a standoff with the West over its nuclear programs.

"We may face no greater challenge from a single country than from Iran, whose policies are directed at developing a Middle East that would be 180 degrees different than the Middle East we would like to see developed," Rice said at a congressional hearing.