Conflict between U.S. partners in Iraq could lead to a resurgence of the Islamic State terrorist group, a bipartisan group of senators has warned.

Eight lawmakers are urging Iraqi Prime Minister Haider al-Abadi to strike an agreement that would end a crisis between the central government and the Kurds, who voted to secede from Iraq and form their own state. The tensions put the United States in a difficult position because American officials are working to strengthen the Iraqi government, while the Kurds played a critical role in expelling ISIS from the country.

“We would like to take this opportunity to stress our belief that the fight against ISIS is far from over,” Sen. Ron Johnson, a Foreign Relations Committee member and chairman of the Homeland Security panel, wrote in a Thursday letter to Abadi obtained by the Washington Examiner. “A rift between Iraqi Kurds and the rest of the country will only exacerbate an already difficult situation. Stable, peaceful, and productive relations between the [Kurdish Regional Government] and the Iraqi government should be a critical priority for both our countries.”

The letter was signed by Johnson and seven other seven other senators, including 2016 Democratic vice presidential nominee Tim Kaine of Virginia and Connecticut Democrat Chris Murphy. Sen. Marco Rubio, R-Fla., Sen. James Risch, R-Idaho, Sen. Cory Gardner, R-Colo., Sen. John Barrasso, R-Wyo., and Sen. Johnny Isakson, R-Ga., also signed the letter.

It's a cautionary note, just weeks after ISIS lost control of Raqqa, its last remaining stronghold in Syria. Mosul, a major Iraqi city that ISIS claimed as its twin capital, fell in a parallel operation by the U.S.-backed coalition that ended in July. But U.S. and Iraqi officials know from experience that a stable Iraq can give way to a terrorist insurgency in a burst of ethnic and sectarian rivalry.

“The protection of Iraq’s Kurdish citizens and minority communities is of the utmost importance to us,” they wrote. “As tensions remain high, we would like to ensure that these communities are protected and to avoid any further violence that could spiral out of control.”

The senators affirmed the Trump administration’s support for peace talks that would keep Iraq whole. That puts them at odds with some prominent lawmakers, such as Senate Minority Leader Chuck Schumer and Texas Sen. Ted Cruz, who endorsed Kurdish independence and denounced Iraqi leaders for coordinating with Iranian-dominated militias in conflict with the Kurds.

"Should the Government of Iraq continue down this path and effectively act as a puppet of Iran, it would require a reevaluation of U.S. support to the country,” Cruz said in October.

Johnson, Kaine, and the others also hope for a rollback of Iran’s influence in the country, particularly where the risk of violence is highest.

“Having invested considerable resources to equip and train the [Iraqi military] we are deeply troubled at the prospect of American-supplied arms being used against Iraqi Kurds or other minority communities,” they wrote.