The office of Secretary General Ban ki-Moon said that it was studying the letter from Mahmoud Ahmadinejad but had no comment.
Mr Ahmadinejad asked the UN to set up a fact-finding team.
Tensions are growing over Iran's nuclear programme, and its rising anger at Washington's nuclear policy.
The fact-finding team requested by Iran's president would investigate the intentions and results of Western military action in Afghanistan and Iraq.
He said that, so far, the invasions had only victimised people in the region, and he declared that US and Nato methods of fighting terrorism had failed.
'State terrorism'
It was not immediately clear what had prompted the letter but it comes amidst rising tensions over Iran's nuclear programme, which the West fears has military aims.
That is something Tehran denies and it is pushing back as the US tries to build momentum for tightening UN sanctions.
Recently, Iran displayed new technology for enriching uranium.
It is also preparing its own nuclear disarmament conference to counter the one hosted by Washington this week.
And on Monday Iran's UN ambassador, Mohammad Khazaee, told a panel here in New York that Washington's new nuclear weapons policy was state terrorism.
US President Barack Obama made clear last week that Iran and North Korea were excluded from new limits on America's use of atomic weapons.
Iranian lawmakers responded furiously to what they saw as a threat of nuclear attack. Iran insists its own nuclear programme is for generating energy, not for building weapons.
