Russian President Vladimir Putin is likely to be killed during a visit to Iran this week.
The spokeswoman, who spoke on customary condition of anonymity, refused further comment.
Interfax news agency, citing a source in Russia's special services, said suicide terrorists had been trained to carry out the assassination.
Putin is to travel to Tehran on Monday night from Germany after meetings with Chancellor Angela Merkel.
During his visit to Iran, Putin is to meet with President Mahmoud Ahmadinejad and attend Tuesday's summit of Caspian Sea nations. He is the first Kremlin leader to travel to Iran since Josef Stalin attended the 1943 wartime summit with Winston Churchill and Franklin D. Roosevelt.
In Tehran, a spokesman for Iran's Foreign Ministry, Mohammad Ali Hosseini, denied any such plot had been uncovered and he characterized the news as disinformation spread by Iran's adversaries.
"These sort of reports are completely baseless and in direction with psychological operations of enemies of relations between Iran and Russia," Hosseini said in a statement.
Hosseini said, "Reporting of this type of sheer lie ... has no news value and cannot harm the planned schedule."
The official Islamic Republic News Agency called the reports part of a psychological war by the Western intelligence services aimed at forcing the cancellation of Putin's visit to Tehran.
Officials have reported uncovering at least two other plots to kill Putin on foreign trips since he became president in 2000.
Ukrainian security officials said they foiled an attempt to kill Putin during a summit in Yalta in August 2000.
In 2001, Russian security officials said a plot to assassinate Putin earlier that year in Baku, the capital of Azerbaijan, had been uncovered by the Azeri special services.
Russian officials linked both alleged plots to Chechen separatists. Putin had sent troops back into the southern Russian republic to crush resistance to Moscow's rule.