Iran's President Mahmoud Ahmadinejad has written to George W Bush proposing "new solutions" to their differences.
The letter comes at a time of acute tension between the US and Iran
The letter will be sent via the Swiss Embassy, which represents US interests in Iran, a government spokesman said.
Mr Ahmadinejad proposes "new solutions for getting out of international problems and the current fragile situation of the world", he said.
Reports say it is the first letter from an Iranian president to a US leader since the Iranian revolution in 1979.
The Swiss have confirmed that they received a sealed A4-sized envelope addressed to President Bush which they would deliver as soon as possible.
Iranian spokesman Gholam-Hossein Elham did not say whether the letter mentioned the nuclear dispute, currently one of the major issues between Iran and the US.
This development comes a day after Iran's parliament threatened to pull out of the nuclear Non-Proliferation Treaty if Western pressure over its programme increases.
A withdrawal would mean the country's programme could no longer be inspected by the UN nuclear watchdog, the International Atomic Energy Agency.
The US has accused Iran of seeking to develop nuclear weapons - a charge Iran strongly denies.
Last week, the US and other states tabled a draft resolution at the UN Security Council calling on Iran to suspend uranium enrichment or face "further action".
Foreign ministers of the council members plus Germany are due to meet in New York on Monday night to discuss how to proceed with Iran.
Iranian foreign ministry spokesman Hamid Reza Asefi said the contents would be made public "at the right time".
US-IRAN TIES: KEY EVENTS
1979: Hostages seized at US embassy in Tehran
1980: Secret US mission to rescue them ends in disaster
1981: Hostages finally freed after intense diplomatic activity
1985/86: Iran-Contra affair, crisis in Reagan presidency
1995: President Clinton imposes sanctions for alleged sponsorship of "terrorism"
2002: President Bush calls Iran, Iraq, North Korea "axis of evil"
April 2006: Iran announces successful uranium enrichment
May 2006: US, UK, France table draft UN resolution calling on Iran to suspend enrichment
The BBC's Frances Harrison in Tehran says that whatever is in the letter is significant because it is the first such high-level communication between Iran and America for almost three decades.
As such it is a bold step by Mr Ahmadinejad, and the timing is key - just as the West is trying to persuade Russia and China to back tough action against Iran, she says.
Mr Ahmadinejad is reinforcing the point that he is willing to negotiate with anyone, including the US president, to avoid conflict over the nuclear issue, our correspondent adds.
The US and Iran have not had diplomatic relations since Washington severed ties with Tehran after Iranian students occupied the US embassy there and took 52 Americans hostage in 1979.
The White House has said it is "unaware" of a letter from Mr Ahmadinejad.