In this picture released by an official website of the office of the Iranian supreme leader, Supreme Leader Ayatollah Ali Khamenei, center, attends a graduation ceremony of army cadets, in Tehran, Iran, Saturday, Oct. 5 2013. Iran's top leader hinted Saturday that he disapproved of the phone call between Presidents Hassan Rouhani and Barack Obama during the Iranian leader's trip to New York last month, but he reiterated his crucial support for the president's policy of outreach to the West. (AP Photo/Office of the Iranian Supreme Leader)

Adding to mounting evidence that President Barack Obama's administration sees Iran as something of a partner in its fight against the Islamic State, the Wall Street Journal reported Thursday that Obama wrote to Iranian Supreme Leader Ayatollah Ali Khamenei last month about the campaign against the Islamic State.

The WSJ reported last week that the U.S. had assured Iran that it would not be targeting the forces of its ally, Syrian President Bashar Assad, as it began striking targets in Syria.

Like that news, the latest revelation will likely stir consternation among many of Obama's current partners in this campaign. From Arab nations that have launched their own strikes against the militant group to moderate U.S.-backed Syrian rebels who are presently besieged by Iranian forces aiding the Syrian regime, key Obama allies see collaborating with Iran as unacceptable because of the country's unwavering support for Assad.

The U.S. has previously concurred with its partner states and the Syrian rebels by identifying Assad as a major cause of the Islamic State's rise to power. By focusing on wiping out the moderate opposition and not targeting the increasingly powerful extremists over the past two years, Assad is perceived as having allowed the militant group to grow to the point where the U.S. and others see it as a major geopolitical risk.

The U.S. did not inform its Middle Eastern allies about its communication with Iran over the Islamic State, the WSJ's sources said.

As a Sunni militant group and the most powerful opponent to Assad and the Shiite-led government in Iraq, the Islamic State threatens Iranian interests. Sources told the WSJ that the president's letter indicated that cooperation with Iran on fighting the Islamic State depends on progress in the ongoing negotiations over Iran's nuclear program.

But the moderate U.S.-backed opposition is afraid of the reverse: that the U.S. might allow its determination to achieve a rapprochement with Iran through a nuclear deal to take priority over the complexities of maintaing its coalition against the Islamic State.

Oubai Shahbandar, a spokesman for the Syrian Opposition Coalition and liaison with rebel commanders fighting the Islamic State on the ground in Syria, told The Huffington Post he hoped this would not be the administration's calculus.

"It is imperative that the administration not value a deal on Iran's nuclear program above ensuring the safety of its partners in Syria," Shahbandar said in an interview last week.

Brookings Institution scholar Suzanne Maloney suggested in a blog post last month that Iran was playing its own game. By positioning itself as a partner against the Islamic State, Maloney argued, Iran hopes "to override any meticulousness on the details [of a nuclear deal], particularly for an Obama administration that is struggling to develop an effective response to regional instability."

The WSJ report flies in the face of administration denials of any coordination with Iran. At an event in Washington on Thursday, Gen. Lloyd Austin, who runs U.S. military operations as the chief of U.S. Central Command, said he was unaware of any cooperation with Iran against the Islamic State in either Iraq or Syria. And the top U.S. negotiator in the talks with Iran, Under Secretary of State Wendy Sherman, said in her last major address that though there was a temptation to collaborate with Tehran given the turmoil in region, the nuclear talks would remain "single-track."

When asked about the reported letter from Obama to Khamenei at a Thursday press briefing, White House press secretary Josh Earnest said there was no military cooperation between the two countries, while noting that they had discussed the Islamic State issue. "[W]e won't share intelligence with them, but their interest in this outcome is something that's been widely commented upon and something that on a couple of occasions has been discussed on the sidelines of other conversations."

"I'm not in a position to discuss private correspondence between the president and any world leader," Earnest said. "I can tell you that the policy that the president and his administration have articulated about Iran remains unchanged."

The administration had previously seemed optimistic about reaching a deal with Iran by the deadline for the nuclear talks, Nov. 24, but multiple sources -- from those speaking to the WSJ to Austin on Thursday -- said it is still unclear whether that goal will be achieved.