"President Ahmadinejad has written a letter to George Bush, which is to be handed to the Swiss embassy," Gholam Hossein Elham, an Iranian government spokesman, told reporters.
It is the first time an Iranian president has been known to officially communicate with an American president since Washington and Tehran cut diplomatic relations in 1980.
"In this letter, while analysing the world situation and finding the roots of the problems, he has proposed new ways for getting out of the existing vulnerable world situation," the spokesman said.
Hamid Reza Asefi, the foreign ministry spokesman, told the ISNA news agency that "once the American president has received the letter, its content will be made public".
President Bush has not ruled out
military action against Iran
The United States and Iran are at loggerheads over Tehran's nuclear programme, which Washington suspects is a cover for ambitions to build atomic weapons. Bush has not ruled out taking military action against the Islamic republic.
Washington accuses Iran of supporting "terrorist" groups across the Middle East.
Washington has not had diplomatic relations with Iran since April 1980, after the seizure of the US embassy in Tehran in 1979 in which 52 Americans were held for 444 days.
A source in Ahmadinejad's office told AFP the letter would be handed to the Swiss embassy on Monday.
The Swiss government assumed the representation of US interests in Iran in 1981, and has since been acting as a conduit for messages between the two arch-enemies.
Diplomacy with Turkey
On Monday, Iran's national security chief urged the West to prefer the International Atomic Energy Agency (IAEA) to the UN Security Council as the forum to find a solution to the crisis over Tehran's controversial nuclear programme.
"We want a political solution to the nuclear question," Ali Larijani said on arrival at the Ankara airport.
"We believe the IAEA is the forum where a solution can be found... and we do not want it bypassed in favor of the UN Security Council," he said.
Larijani began his one-day visit here with a visit to his official host, Yigit Alpogan, secretary-general of Turkey's National Security Council.
Yigit Alpogan (L) and Ali Larijani
meet in Ankara on Monday
Turkey, Iran's western neighbor, has repeatedly exhorted Tehran to be more open with the international community about its nuclear programme, and Larijani reiterated that "we remain transparent."
Once military rivals for regional supremacy, Turkey and Iran have been at peace for nearly 370 years; their borders have remained unchanged since and they have been allied in a number of regional pacts over the decades.
Turkey imports natural gas from Iran via a pipeline that became operational in 2001 under a 25-year deal signed in 1996.
The agreement foresaw that Turkey would import four million cubic meters (140 million cubic feet) of Iranian natural gas in 2002, with the amount gradually increasing to peak at 10 million cubic meters (350 million cubic feet) in 2007.