Iranian President Hassan Rouhani is warning that regional oil supply could be jeopardised if the United States convinces its allies to stop buying Tehran's oil.

Rouhani spoke to Iranian expatriates in Switzerland during his visit there on Monday, and Iranian state TV broadcast his remarks.

He says the US has threatened to stop allowing Iran to export its oil.

Rouhani didn't elaborate but when pressured in the past, Iran has threatened to close the strategic Strait of Hormuz though which one third of the world's oil supply passes.

Iran exports more than two million barrels of oil a day, making it OPEC's second-largest crude exporter.

After the US pulled out of the nuclear deal with Iran, Washington has been pushing allies to cut oil imports from Iran altogether by November.