The supreme leader of Iran, Ayatollah Ali Khamenei, has accused the US of promoting sectarian strife in Iraq.
Ayatollah Ali Khamenei also criticised Jordan's King Abdullah II
Speaking at Friday prayers in Tehran, Ayatollah Khamenei said it was US policy to pit Sunni Muslims against Shias "to sow pessimism".
Iraq has been torn by sectarian violence since the bombing of a revered Shia shrine in February.
The US accuses Iran of supporting Shia insurgents in Iraq, an accusation Tehran has denied.
In a speech broadcast live on television, Ayatollah Khamenei said: "Making Sunnis and Shias suspicious of one another... is the policy of the Americans in Iraq.
"Our Iraqi brothers need to stand united... and beware that the enemy plans to turn people against the people."
The US has accused Iran of funding Shia insurgents to infiltrate Iraq.
Iran says it has no interest in creating instability in its neighbour.
In his speech, Ayatollah Khamenei also accused Jordan's leader King Abdullah II of being a "dependent" spokesman for the US.
King Abdullah, a Sunni, accused Iran in 2004 of trying to create a "Shia crescent" of power from Iran to Lebanon.