Iran warned on Sunday it will react strongly against any US intrusion into its airspace after two of its warplanes fired at an American drone 10 days ago, the ISNA news agency reported.

"Yes, we opened fire, and it was with warning shots. If they do it again they can expect an even stronger response," the agency quoted General Amir-Ali Hadjizadeh, head of the elite Revolutionary Guards air and space forces, as saying.

Pentagon spokesman George Little said on Thursday the Iranian Su-25 Frogfoot fighters fired at the robotic Predator drone on November 1 but did not hit it.

"They intercepted the aircraft and fired multiple rounds," he said.

The American drone was "never in Iranian airspace" and came under fire from the fighter jets off the Iranian coast over international waters, Little said.

In a warning to Tehran, the Pentagon spokesman said the United States was prepared to safeguard its forces.

"We have a wide range of options, from diplomatic to military, to protect our military assets and our forces in the region and will do so when necessary," Little said.

On Friday, Iranian Defence Minister Ahmad Vahidi confirmed the incident, saying the drone had "entered the space over the territorial waters of the Islamic Republic of Iran in the Persian Gulf area."

Hadjizadeh was reported as saying on Sunday: "This spy drone entered Iranian airspace and had to turn around because of the immediate reaction by fighters of the Revolutionary Guards."

He added that the US unmanned aircraft had been "flying over Kharg island to gather information about economic activity on the island, and the arrival and departure of oil tankers."

Kharg, 25 kilometres (15 miles) off the Iranian mainland, is the Islamic republic's main export terminal for its oil.

The United States and the European Union have both imposed economic sanctions against Iran over its controversial nuclear ambitions which Western powers and Israel believe are aimed at producing atomic weapons.

Tehran denies the charge, sayings its programme of uranium enrichment is entirely peaceful and is for energy and medical purposes.

The November 1 drone incident came less than a week before America's presidential election, but the Pentagon kept it quiet until reports of the confrontation leaked out.