Iran has criticised planned US military exercises in the Gulf as provocative.
Ayatollah Khamenei is Iran's most senior political and religious leader
Iran's official news agency IRNA quoted an unnamed foreign ministry official as describing the military manoeuvres as dangerous and suspicious.
Reports say the US is to hold naval exercises at the end of October with Bahrain, Kuwait, France and Britain.
Separately, Supreme Leader Ayatollah Ali Khamenei has called for Muslim unity to deal with what he called the "dirty aims" of the US and Israel.
Speaking at prayers to mark the festival of Eid al-Fitr at the end of Ramadan, Ayatollah Khamenei said the Middle East had turned a page in its history after what he described as the victory of the militant Lebanese group Hezbollah in its recent conflict with Israel.
In particular he called on Palestinian factions and groups to remain united, saying their enemies wanted to sow the seeds of division among them.
BBC Tehran correspondent Frances Harrison says the factional fighting between Fatah and Hamas has dismayed Iranian leaders, who fear it could diminish support for the Palestinian struggle.
Reports say the US-led naval exercises based near Bahrain will practise intercepting and searching ships carrying weapons of mass destruction and missiles.
The Iranian foreign ministry official said the US-led exercises were not in line with the security and stability of the region. Instead, they are aimed at fomenting crises, he said.
He complained that it was the warmongering of neo-conservatives in America who want to win the mid-term US congressional elections in November.
The manoeuvres come as America is pushing for tough UN sanctions on Iran, prohibiting nuclear cooperation or sales of ballistic missiles.
Our correspondent says it is not yet clear whether the UN would authorise the searching of ships heading to Iran.