TEHRAN (Reuters) - Iran demanded consular access on Sunday to a man held in the United States over a suspected plot to kill the Saudi ambassador and vowed to respond robustly to any "inappropriate measure" by the West.

Manssor Arbabsiar, who holds U.S. and Iranian citizenship, was arrested in September over the plot which Tehran called a fabricated "comedy show" but which U.S. lawmakers said was "very real" and showed the need for tougher sanctions on Iran.

"Any inappropriate measure against Iran, whether political or security-related, will be strongly confronted by the Iranian nation," Supreme Leader Ayatollah Ali Khamenei said, accusing Washington of inventing the plot to divert attention from the "Occupy Wall Street" protests.

U.S. authorities announced the plot last Tuesday, saying Arbabsiar, a naturalized U.S. citizen, had paid a U.S. undercover agent posing as a Mexican drug cartel hit man to assassinate Saudi Ambassador Adel al-Jubeir.

Many experts, and some Washington officials, expressed doubts over the plot, which even the head of the FBI said sounded like a Hollywood script, but the heads of the intelligence committees in the U.S. Congress appeared on television Sunday to say it should be taken seriously.

Senate Intelligence Committee Chairwoman Dianne Feinstein said she was initially skeptical when first briefed about the plot in September but now believed "it's very real."

"Our country should not be looking to go to war," Feinstein told the "Fox News Sunday" program. "We should be looking to stop bad behavior, short of war.

President Barack Obama said Thursday that Iran -- already at odds with Western governments over its nuclear program -- would face the toughest possible sanctions and the United States would not take any options off the table, the standard code to refer to possible military action.

Iran's Foreign Ministry summoned the Swiss charge d'affaires who represents U.S. interests in the country that broke ties with Washington shortly after the 1979 Islamic revolution.

"There is no doubt regarding the baselessness of the U.S. allegations," a ministry official told the Swiss representative, according to state broadcaster IRIB.

"However, providing personal information about the accused and consular access to him is among the duties of the U.S. government. Any delay in that respect would be in contravention of international law and the U.S. government's responsibilities," the unidentified official said.

Iran's diplomatic interests in the United States are handled by an office in the Pakistani embassy.

U.S. officials have mooted the possibility of upping sanctions on the Central Bank of Iran -- a move which could make it harder for Tehran to receive payment for its oil exports, a vital source of hard currency.

House of Representatives Intelligence Committee Chairman Mike Rogers said Washington should be pushing for tighter sanctions.

"Put pressure on the Chinese and the Russians and say, listen, you're either going to stand with the nation that is engaged in nation-state terrorism or you're going to stand with the rest of the international community," he told ABC's "This Week."

(Writing by Robin Pomeroy)