US President Barack Obama has secretly written a letter to Iran's Supreme Leader Ayatollah Ali Khamenei urging progress on nuclear talks and stressing common interest in fighting the Islamic State of Iraq and the Levant (ISIL), the Wall Street Journal says.

Obama sent the letter to Khamenei in mid-October, according to the report published on Thursday citing unnamed White House officials.   

"Mr Obama stressed to Mr Khamenei that any cooperation on [ISIL] was largely contingent on Iran reaching a comprehensive agreement with global powers on the future of Tehran's nuclear programme by a November 24 diplomatic deadline," the report said.

It also noted that it was at least the fourth time Obama had written to Khamenei since taking office in 2009.

Carol Lee, one of the journalists behind the report, told Al Jazeera the letter shows just how much officials in the White House believe Khamenei is key to achieving a deal. 

"They can talk to [Mohammad Javad] Zarif, the Iranian foreign minister, and others within the government but its really the Supreme Leader who holds the keys to whether or not they get this done," Lee said.

Obama said on Wednesday that his administration has put forward a "framework" plan for a nuclear agreement with Iran. But he warned it was unclear if a deal would be struck by the November 24 deadline.

"We presented to them a framework that would allow them to meet their peaceful energy needs," Obama said.

It was the first time the US has alluded to a completed framework being on the table, and came just days before John Kerry, US secretary of state, holds fresh negotiations with his Iranian counterpart.

'Verifiable deal'

If Iran is sincere in not seeking a nuclear weapon, "if that's in fact true, they have an avenue here to provide that assurance to the world community", Obama said.

It would be "a progressive step-by-step verifiable way" which would "allow them to get out from under sanctions so they can re-enter as full-fledged members of the international community".

Iranian leaders have long insisted they are not seeking to develop an atomic bomb, saying the country's nuclear programme is solely for peaceful civilian energy needs.

But Obama again repeated the US administration's long-held insistence that "no deal is better than a bad deal".

"Whether we can actually get a deal done, we'll have to find out over the next three to four weeks," Obama said before weekend talks in Oman between Kerry and Zarif.

According to The New York Times this week one part of the deal could involve shipping Tehran's huge stockpile of uranium to Russia to be converted into fuel rods for Iran's Bushehr nuclear plant.

Once uranium has been converted into fuel rods, it is difficult to use it as a weapon.

Under an interim deal reached a year ago, Iran agreed to halt uranium enrichment and even reduce some of its stockpile in exchange for the unblocking of about $7bn in frozen oil revenues to help its economy.

Source: Al Jazeera and agencies