Shi'ite Iran has been alarmed by rapid territorial gains made in Iraq by the militants of the 'Islamic State of Iraq and the Levant' (ISIL), which seeks a caliphate ruled on mediaeval Sunni Muslim precepts in Iraq and Syria.
ISIL's advances pose a threat to the survival of Shi'ite-majority Iraq as a united country. Iraqi Prime Minister Nuri al-Maliki's Shi'ite-led government, an ally of Tehran, has called for military help from the United States, Iran's longtime foe.
A message posted in English on the Twitter account @khamenei_ir said the Sunni militants wanted to foment distrust between Sunni and Shi'ite Muslims, a goal they shared with "arrogant" powers - normally an Iranian codeword for the United States and its Western and Israeli allies.
The message referred to "takfiri" militants - Sunnis who proclaim followers of other sects of Islam to be infidels and therefore legitimate targets of holy war.
"Muslims should be aware of Takfiris and arrogant's common goal to create a war in Muslim world - both Shias and Sunnis should be vigilant," it said.
"The great danger of the Takfiris is not about killing innocents, although that's a crime, a big crime.
"Yet greater danger of Takfiris is to make Shias and Sunnis distrust one another; this is a really great danger which fulfils goal of arrogants."
Iran's regional rival, Sunni powerhouse Saudi Arabia, has also expressed alarm about this month's developments in Iraq and the dangers they pose for Middle East security and peace.
© Thomson Reuters 2014