The United States and Iran are both helping fight Sunni militants in the Islamic State group. But one of the biggest battlefields is Syria, where Washington and Tehran back opposing sides in the civil war.
The United States and Iran are both backing Kurdish fighters in Iraq who have retaken some ground near the Syrian border with help from coalition airstrikes against Islamic State fighters, who are also known as Daesh.
Meeting with Arab allies, U.S. Secretary of State John Kerry said there is broad support for the fight.
“Every country in the region is opposed to Daesh, without exception. Whether it's Iran or Lebanon or Syria itself or Turkey or Iraq, Saudi Arabia, the Gulf states -- they're all opposed,” said Kerry.
In Syria itself, though, it is less clear whom best to back against the Islamic State, said Nora Bensahel from the Center for a New American Security.
“Iran faces a very difficult calculation about exactly what its interests are, just as the United States does,” says Bensahel.
Bensahel, who is also a professor at Georgetown University, says Washington and Tehran are both sorting through a broad mix of on-the-ground forces.
“The problem is that in Syria everything is so interconnected that it’s not as simple as the enemy of my enemy is my friend. The enemy of my enemy might still be my enemy on a whole lot of other things,” says Bensahel.
The Obama administration is determined to keep its contacts with Tehran focused on rolling back its nuclear program. But there have been direct talks on Syria and the Islamic State.
Assad's Tehran support
Iranian National Security Council Director Ali Shamkhani said Tehran stands steadfast against more direct foreign intervention.
"There will not be an intention of a ground intervention because they failed in Iraq and Afghanistan, and we oppose any foreign intervention in the region by these states," said Shamkhani.
Tehran will also not allow the fight against the Islamic State group to weaken Syrian President Bashar al-Assad, said American University professor Akbar Ahmed.
"Iran has a very aggressive foreign policy. It's not going to just allow events to take place without having a say in them or attempting a say in them," said Ahmed.
U.S. officials say one area of agreement with Iran is the fight for Kobani where there are no Syrian government forces and both Washington and Tehran support the Kurdish opposition.