The U.S. and Iran were once on the opposite side of the Middle East war, but with the Islamic State extremists in Iraq and Syria now their common enemy, U.S. President Barack Obama made the rare move to write a secret note to Iran Supreme Leader Ayatollah Ali Khamanei.

The U.S.-led Coalition against the Islamic State excludes Iran, which is battling the extremist militant group on the ground level, while the U.S. leads the air campaign.

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Iran's action is to show its support for embattled Syrian President Bashar Assad, whom the U.S. seeks to oust from power because of the alleged massive human rights abuses in that country.

The move by Obama is linked to the upcoming Nov. 24 deadline of nuclear talks among the U.S., Iran and five other world powers, namely Britain, China, France, Germany and Russia. The agenda of the talks include Iran reducing the number of its uranium and redesigning a heavy water reactor to make it incapable of plutonium production, used in nuclear warheads.

Iran insists that the aim of its nuclear program is for peaceful purposes, but the U.S. and other nations believe that Iran wants to build a bomb.

While Washington is not ruling out a possible nuclear deal with Iran, which could lead further discussions on other issues, Obama said on Wednesday that "prospects for final agreement remain uncertain" since the ability to secure the deal is an "open question."

The resumption of the international nuclear negotiations is the result of a series of closed-door meetings between the U.S. and Iran in 2013. But Obama stressed that Washington will not coordinate its military action with Tehran.

"The United States will not cooperate militarily with Iran in that effort ... We won't share intelligence with them," Sun Herald quotes White House spokesman Josh Earnest, who declined to comment on the letter first reported on Thursday by The Wall Street Journal.

The spokesman said at a White House briefing that he is not in a position to discuss private communication between Obama and world leaders.

"I can tell you that the policy that the president and his administration have articulated about Iran remains unchanged," Reuters quoted Earnest.

The Associated Press vouched that the rare Obama letter exists, based on a separate confirmation by the news agency's diplomatic sources who gave the information on the condition that they remain unnamed because of lack of authority to discuss the matter publicly.

TagsBarack Obama, secret letter, U.S.-Iran, Ayatollah Ali Khamanei, ISIS