Answering a question from RIA Novosti, FBI spokesman Stephen Kodak neither confirmed nor denied his agency's involvement, but said that according to the standard procedure for passing information from one state to another, such a report would have come from the U.S. State Department.
Russia's anti-terrorism committee, headed by Federal Security Service's (FSB) boss Nikolai Patrushev, said Tuesday the service had obtained a report from foreign sources of a possible terrorist plot to attack ground transport and one of the country's subway systems. However, he gave no indication of what country the warning had come from.
Russia's international cooperation on anti-terrorism involves contacts with many countries, including the United States.
Kodak said the FBI never comments on information exchange with foreign intelligence services, and added that it is up to its foreign partners to decide whether to disclose sources.
While the report is being verified, Patrushev has issued orders to federal and regional anti-terrorism headquarters to keep on high alert and take all necessary search and preventive measures.
The country's main subway is in Moscow, but Russia also has metro systems in several other major cities.
Anti-terrorist policing has been stepped up in the Moscow Metro, and Wednesday morning reports said the FSB's regional departments throughout Russia have been put on high alert to forestall any attacks.
The national security service has also recommended that heads of passenger transport companies take pains to ensure people's security, and advised passengers to be vigilant and inform police of anything suspicious.
Two bomb explosions in Moscow's subway system in 2004 killed a total 49 people and injured more than 300. A bomb detonated in a train carriage between the central metro stations Avtozavodskaya and Paveletskaya in February, and in August another went off near the entrance to Rizhskaya station in northern Moscow.