Iran's supreme leader Ayatollah Ali Khamenei on Monday warned regional countries of the "high price" they would eventually pay for supporting extremist Sunni Muslim groups fighting Tehran's key ally Syria.

Shiite Iran is a staunch supporter of President Bashar al-Assad's regime in its struggle against mostly Sunni rebels backed by Arab nations and Western powers.

Khamenei made the comments during a meeting with the visiting Kuwaiti Emir Sheikh Sabah al-Ahmad Al-Sabah whose trip to Tehran has been widely seen as an attempt to warm up the frosty ties between Iran and Sunni-ruled monarchies in the Gulf.

"Unfortunately, some regional countries do not take heed of the danger of Takfiri groups which will threaten them in future and they are still backing these groups," the leader's website khamenei.ir reported.

Takfiri is a synonymous term Iranian officials use for Sunni extremists.

"Some of the regional countries are backing the Takfiri groups and supporting their massacres and crimes in Syria," said Khamenei.

"Eventually these countries will be forced to eradicate these extremists, with a high price," Khamenei added.

Syrian opposition groups and Western countries have accused Tehran of supplying Damascus with military and financial backing.

Iran says it only has military experts on the ground in Syria and that it is providing the Assad regime with humanitarian assistance.