Washington (AFP) - The United States and Iran have a "mutual interest" in defeating the Islamic State group but the long-time foes are not cooperating to do so, Secretary of State John Kerry said Wednesday.

"They are totally opposed to ISIL and they are in fact taking on and fighting and eliminating ISIL members along the Iraqi border near Iran and have serious concerns about what that would do to the region," Kerry told lawmakers, referring to IS by another acronym.

"So we have at least a mutual interest, if not a cooperative effort."

Kerry, who has been pivotal to Washington's drive to strike a deal to curb Iran's nuclear ambitions, said the United States had not asked Tehran to get involved in the fight against the IS group.

Since last year, the Sunni extremist group has waged a brutal offensive in Iraq and Syria, grabbing land and carrying out executions and forced religious conversions.

Washington forged a coalition of Western and Arab nations to confront the extremists, but mainly Shiite Iran is not officially part of the group.

In December, Kerry also denied there was any military coordination with Iran, after the Pentagon said that Iranian jets had deployed against IS fighters in eastern Iraq.