Moscow had already agreed the sale of the missiles, part of an air defense system that observers say would endanger Israel or the United States' ability to carry out air strikes against Iranian targets.
But the delivery has been delayed by Western pressure, and would be forbidden outright if Washington convinces the UN Security Council -- including Russia -- to approve a new round of sanctions.
"The paragraph of the resolution on the ban on arms sale to Iran includes several categories of weapons, including defensive weapons," said one diplomat.
"If it's adopted, the resolution would include the Russian S-300s and would prevent these arms from being delivered."
Another diplomat, also speaking on condition of anonymity, agreed.
"The supply of the S-300 would indeed be prohibited by this text, if it is adopted in its current form," he said.
Diplomats said the text for new sanctions, designed to force Iran to abandon a nuclear program that the West fears will lead it to build nuclear arms, had been broadly agreed.
The five permanent members of the UN Security Council -- Britain, China, France, Russia and the United States - are now discussing appendices to the text that will go before the full body, they said.
According to a copy of the draft, seen by AFP, the sanctions would ban the sale of tanks, armored fighting vehicles, large caliber artillery, war planes, attack helicopters, warships, missiles and missile defense systems to Iran.