Two men toting a briefcase stuffed with false bond certificates purportedly worth trillions of euro tried to bluff their way into the exclusive Vatican bank in a foiled fraud plot, Italian police said.

Financial Guard police Lt Col Davide Cardia said the would-be swindlers, who were wearing business suits, tried to convince Swiss Guards at a Vatican City gate earlier this month that "cardinals were expecting them".

Mr Cardia told the Associated Press that the suspects, a middle-aged Dutchman and a US citizen, were detained by Vatican authorities after rapid checks by Vatican officials showed they had no such appointment nor connections with the Institute for Religious Works, the formal name of the bank, which is behind the tiny city-state's walls and is not open to the public.

The Vatican has been scrambling to upgrade procedures and standards at the bank since a 2010 money-laundering probe.

Mr Cardia said the fake documents purported to be bond certificates for non-Italian companies.

"The sum - worth some three trillion euro - is impressive, even though it's only symbolic because we're talking about false" certificates, said Mr Cardia, in charge of the financial police's operations in Rome and the surrounding area.

Investigators suspect the men might have planned to use the fake bonds as security to open a hefty line of credit through the Vatican bank.

The Vatican asked Italian authorities to help in the investigation.

Italian police searched the men's room at a hotel near the Vatican and seized stamps and seals used to create the false documents, Mr Cardia said.

Both suspects, whose names were not released by police, had been previously investigated for attempted fraud in Asian countries, Mr Cardia said without elaborating. They were issued citations and released on their own recognisance pending further investigation, since Italian law does not require arrest for investigation of attempted fraud, according to the official. Both are believed to have left Italy.