President Obama has allegedly sent a secret letter to Iran's Supreme Leader stressing a shared interest in fighting Islamic State extremists in Iraq.

The President sent the secret letter last month, according to The Wall Street Journal.

The letter reportedly said cooperation between the US and Iran on combating the extremist group was tied to a deal being reached on Iran's nuclear programme.

On Thursday, White House spokesman Josh Earnest declined to discuss whether the private correspondence took place, adding that the President's policy on Iran has not changed.

Many commentators have taken this as a tacit confirmation of the letter's existence.

Mr Earnest, who said the US has discussed the IS campaign with Tehran on the sidelines of nuclear negotiations, previously said the two issues are "entirely separate".

"The United States will not cooperate militarily with Iran in that effort," he said of the fight against IS.

"We won't share intelligence with them. But their interests in the outcome is something that's been widely commented on - commented upon and something that on a couple of occasions has been discussed on the sidelines of other conversations."

Speaking to reporters on Wednesday, Mr Obama said it was an "open question" as to whether international negotiators and Iran can reach a deal over Tehran's nuclear programme.

The President said the next few weeks will be key ahead of the looming 24 November deadline.

"Whether we can actually get a deal done, we're going to have to find out over the next three to four weeks," Mr Obama said.