The U.S. effort to stem Islamic State’s reign of terror in Syria and Iraq has America and one of its most bitter enemies - Iran - in the same camp, which some experts warn could be a recipe for disaster.

By some estimates, Iran and its proxies are responsible for the killing of as many as 1,000 Americans since 2003, but both nations are seeking to stop the spread of the Sunni-dominated  Islamic State, which has killed U.S. citizens and threatened attacks on America and has angered Iran by attacking Shia shrines and mosques. With the Obama administration seeking help from Iran’s regime, Middle East experts are raising alarms.

“Iran is at war with us,” Robert Caruso, a former Department of Defense official and an expert on Tehran's military operations, told “Iran is actively subverting American leadership throughout the world.”

Last week, State Department spokeswoman Jen Psaki said there were “diplomatic discussions” with Iran on the issue, but added, “…we are not and will not coordinate militarily.”

Yet Reuters reported on Tuesday that a senior Iranian official said the U.S. gave Iran advance notice about the U.S. and Arab countries’ military strikes in Syria.

"This issue was first discussed in Geneva and then was discussed thoroughly in New York where Iran was assured that Assad and his government will not be targeted in case of any military action against Daesh (Islamic State) in Syria," the Iranian official told Reuters.

Iran’s ties to Al Qaeda raise new questions about whether it is prudent military strategy to coordinate with Iran. U.S. military strikes hit the Al Qaeda group Khorasan in Syria this week and there are unconfirmed reports its leader, Muhsin al Fadhli, could have been killed. Al Fadhli was the former chief of Al Qaeda's Iran-based network as the Islamic republic reportedly provided refuge for Al Qaeda terrorists as part of a secret alliance.

Caruso cited Iran’s actions in Iraq against the U.S. during the years 2003 to 2011 as a sign of its  belligerence, including its supply of powerful armor-piercing bombs to Sunni and Shiite groups fighting American personnel.

Phillip Smyth, a researcher on Iranian proxies affiliated with the University of Maryland, told the “summary executions“ of five American soldiers in Karbala, Iraq, in January 2007 were the handwork of Iranian Revolutionary Guard Corps (IRGC) and a Hezbollah operative. The Lebanese-based Hezbollah organization is a strategic partner of the Tehran regime.

Estimates of the number of Americans murdered by Iran’s terror apparatus and its proxy system in Iraq range from 100 to as many as 1,000, Smyth said.

Commenting on the role of the Iranian Revolutionary Guards’ Quds force, Gen. Martin Dempsey, chairman of the U.S. Joints Chief of Staff, told NPR: “I can tell you there’s a lot of leaders of the [force] and some of the advisers to the Shiite militia that have a lot of American blood on their hands.”

And former CIA Director Michael Hayden explicitly stated during his tenure in 2008,“It is the policy of the Iranian government, approved to the highest levels of that government, to facilitate the killing of Americans in Iraq.”

The Hezbollah Shiite senior commander Ali Musa Daqduq, who played a critical role in the murders of the five Americans soldiers in Karbala, was captured in March 2007 by U.S. forces. Iraq’s pro-Iranian government released him in 2013 over objections from the Obama administration. Critics slammed the Obama administration at the time for failing to both block the release of Daqduq and prosecute him for terrorism.

Daqdug rejoined Hezbollah—a U.S.-designated terrorist organization—and is back in Lebanon with the Shiite militia.

Eyebrows were raised last week when Secretary of State John Kerry said at a United Nations Security Council meeting on stopping the Islamic State’s aggression, “There is a role for nearly every country in the world to play, including Iran.”

The U.S. State Department classifies Iran as a state-sponsor of terrorism. Kerry’s overture to Tehran comes on the heels of Iran’s Supreme Leader Ayatollah Ali Khamenei rejection  via his Twitter feed on Sept. 15 of a “U.S. offer to Iran’’ on defeating Islamic State as part of a broad-based coalition.”

Given their bitter history, the idea the two nations could work together to fight a common enemy is folly, said Julie Lenarz, executive director of the London-based Human Security Center.

“Some people inside the Obama administration seem to think that Iran could be a legitimate partner for the U.S. in the fight against Islamic State in Syria and Iraq," Lenarz said. "This strategy, however, is a gross miscalculation of the crisis as well as an expression of ignorance of the overall situation in the Middle East.”

For U.S. allies in the region, the rise of Iran's role is troubling.

“The Iranians are getting almost everything but giving almost nothing," Israel’s Strategic Affairs Minister Yuval Steinitz said. "Although it is important to defeat ISIS, if Iran gets nuclear weapons, it’s a different world for decades. This is the main threat to global security and should be the priority.”

Benjamin Weinthal reports on human rights in the Middle East and is a fellow at the Foundation for Defense of Democracies. Follow him on Twitter @BenWeinthal