Iran denies the claims and says the US-led invasion of Iraq in 2003 has fomented bloodshed between Iraq's majority Shia and minority Sunni Arabs.
Khamenei's speech, which marked the first Friday prayer of the Islamic month of Ramadan, was attended by thousands of worshippers.
"America's power in the region is waning," he said. "[The US is] looking forward to pulling out of Iraq."
The US severed diplomatic ties with Tehran shortly after the 1979 Islamic revolution which forced the exile of Mohammed Reza Pahlevi, the US-supported shah.
The two countries have held talks in Baghdad in recent months to address security concerns in Iraq but relations remain very tense.
Khamenei said a "psychological war" had been conducted against Iran, and also criticised the Arab-Israeli peace process, Israel and some European countries.
Washington also remains suspicious over Iran's nuclear programme. The US says Iran is trying to build nuclear weapons.
Iran denies the US' allegations on its uranium enrichment work, saying its nuclear technology is being used to generate electricity.
World powers are set to meet in Washington on September 21 to discuss a new resolution on the nuclear programme.
The Bush administration has outlined unilateral measures to pressure Iran, including sanctions on the Guards' Quds force, which Washington says is fuelling violence in Iraq.
In his speech, Khamenei said Iran would not yield to any sanctions, saying that Tehran had resisted such moves since the 1979 revolution.