"In the 75 years of United Nations history, America has never been so isolated," foreign ministry spokesman Abbas Mousavi tweeted hours after just two of the Council's 15 members backed a US resolution to extend the embargo.

"Despite all the trips, pressure and the hawking, the United States could only mobilise a small country (to vote) with them," he added, in reference to failed efforts by US Secretary of State Mike Pompeo to rally support for a US resolution.

Iranian state television's website said the only other country to back the US resolution was the Dominican Republic, citing unnamed sources.

The embargo on conventional arms sales to Iran is due to expire on October 18 under the terms of a resolution that blessed the 2015 nuclear deal between the Islamic republic and world powers.

"Last night, Iran's active #diplomacy, along with the legal force of (the nuclear deal), defeated the United States again at the Security Council," Mousavi tweeted.

President Donald Trump unilaterally withdrew the United States from the nuclear deal in 2018, escalating tensions between the longtime enemies.