WASHINGTON (Reuters) - Vice President Joe Biden said on Tuesday the United States is concerned that Iran is becoming more of a military dictatorship and insisted that Tehran will not be permitted to acquire a nuclear weapon.

Biden also said he was unsure how much power hardline Iranian President Mahmoud Ahmadinejad wields during an interview with MSNBC's "Hardball" program conducted while he was in Jerusalem on a Middle East tour.

"The question is, we're not even sure what he controls," Biden said of Ahmadinejad. "The question, is how much of the security apparatus does he control day to day? There's a lot we don't know."

Biden added: "There is some concern that ... this is becoming more of a military dictatorship. But the truth is, no one knows for certain."

The United States has not ruled out any options for dealing with Iran, including the military option, but is emphasizing diplomacy in its long-running feud with Tehran and is seeking support from the U.N. Security Council for tougher sanctions.

Washington and its allies accuse Iran of seeking to develop a nuclear weapon, a charge denied by Tehran.

"We are doing everything that is within our power, and we will do everything, to prevent Iran from getting a nuclear weapon, having the capability to use a nuclear weapon," Biden said.

Washington believes sanctions are the best way to avoid conflict and destabilizing arms races.