U.S. and Iraqi officials say Islamic State fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country's fragile security.

Hundreds — likely more than 1,000 — militants have crossed the open, desert border in the past six months, defying a massive operation by U.S., Kurdish, and allied forces to stamp out the remnants of the jihadi group in eastern Syria.

That's according to three Iraqi intelligence officials and a U.S. military official. The officials spoke on condition of anonymity because they were not authorized to comment publicly on intelligence matters.

FILE - In this Tuesday, Feb. 19, 2019 file photo, a U.S.-backed Syrian Democratic Forces fighter reacts as an airstrike hits territory still held by Islamic State militants in the desert outside Baghouz, Syria. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months. (AP PhotoFelipe Dana, File)

FILE - In this Tuesday, Feb. 19, 2019 file photo, a U.S.-backed Syrian Democratic Forces fighter reacts as an airstrike hits territory still held by Islamic State militants in the desert outside Baghouz, Syria. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months. (AP PhotoFelipe Dana, File)

But indications of the extremist group's widening reach in Iraq are clear — from kidnappings, assassinations and roadside ambushes aimed at intimidating the local population and financing the group's extortion rackets.

FILE - In this Saturday, Feb. 16, 2019 file photo, U.S.-backed Syrian Democratic Forces fighters take a break as the fight against Islamic State militants continues in the village of Baghouz, Syria. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months, defying a massive operation by U.S., Kurdish, and allied forces to stamp out the remnants of the jihadi group in eastern Syria. (AP PhotoFelipe Dana, File)

FILE - In this Saturday, Feb. 16, 2019 file photo, U.S.-backed Syrian Democratic Forces fighters take a break as the fight against Islamic State militants continues in the village of Baghouz, Syria. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months, defying a massive operation by U.S., Kurdish, and allied forces to stamp out the remnants of the jihadi group in eastern Syria. (AP PhotoFelipe Dana, File)

 FILE - In this Feb. 1, 2019 file photo, a flag of the Islamic State group and some of the weapons they used in battle, are on display in the Popular Mobilization Forces War Museum in Baghdad, Iraq. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months. (AP PhotoKhalid Mohammed, File)

FILE - In this Feb. 1, 2019 file photo, a flag of the Islamic State group and some of the weapons they used in battle, are on display in the Popular Mobilization Forces War Museum in Baghdad, Iraq. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months. (AP PhotoKhalid Mohammed, File)

 File - In this Monday, Feb. 19, 2018 file photo, mourners pray over the flag-draped coffins of two fighters of the Popular Mobilization Forces, killed during fighting against Islamic State militants, during funeral procession in Najaf, Iraq. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months. (AP PhotoAnmar Khalil, File)

File - In this Monday, Feb. 19, 2018 file photo, mourners pray over the flag-draped coffins of two fighters of the Popular Mobilization Forces, killed during fighting against Islamic State militants, during funeral procession in Najaf, Iraq. U.S. and Iraqi officials say IS fighters facing defeat in Syria are slipping across the border into Iraq, where they are destabilizing the country’s fragile security. Hundreds -- likely more than 1,000 -- IS fighters have crossed the open, desert border in the past six months. (AP PhotoAnmar Khalil, File)