Tehran (AFP) - Iran's supreme leader Ayatollah Ali Khamenei launched a fierce attack against regional rival Saudi Arabia, saying it was being pumped "like a milking cow" by "infidel" Americans.

"These people (the Saudis) appear to believe in the Koran... but in practice they act against its teachings," Khamenei said at a meeting on Saturday to mark the start of the holy month of Ramadan.

"They are close with the infidels and offer the enemy the money they should be using to improve the lives of their own people.

"But in reality there is no closeness and, as the Americans have said, they are just there to pump them for money like a milking cow, and later slaughter them," he added.

During a visit by President Donald Trump to Riyadh on May 21, Saudi Arabia agreed to buy $110 billion of US weapons and signed investment deals worth billions more.

Sunni-led Saudi Arabia and Shiite-majority Iran are on opposing sides in a number of regional conflicts, including in Syria and Yemen.

Trump sought to boost an anti-Iran alliance of Arab powers during his trip -- but the visit was dismissed as a "show with no practical or political value" by Iran's newly re-elected President Hassan Rouhani.