Iran has sent the United States a formal diplomatic note disputing U.S. allegations that Tehran was behind an alleged plot to assassinate the Saudi ambassador to the United States, the State Department said on Tuesday.

"It was about seven pages. It was a rant. It was full of all kinds of denials," State Department spokeswoman Victoria Nuland told a news briefing. "There was not a lot new in there from our perspective."

U.S. officials announced last month they had uncovered a plot by two men with links to Iran's security forces to assassinate the Saudi ambassador to the United States by planting a bomb at a Washington restaurant.

One of the men, who allegedly paid a U.S. undercover agent posing as a Mexican drug cartel hitman, has been arrested while the United States says the other is in Iran.

Iran has rejected the accusations, calling them a fabrication by the United States to exacerbate tensions between Saudi Arabia and Iran, and divert attention from U.S. domestic economic problems.

Nuland said the Iranian diplomatic note was handed to U.S. officials by diplomats from Switzerland, which represents U.S. interests in Tehran in the absence of formal diplomatic relations.

The two countries broke diplomatic ties following the 1979 Islamic revolution and the storming of the U.S. embassy in Tehran. Relations remain tense, with Washington accusing Iran of pursuing nuclear weapons and sponsoring terrorism, charges denied by Tehran.