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

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, 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 p...