WASHINGTON (AP) — The battle-hardened Army commander nominated to take over U.S. military operations in Afghanistan warned senators Tuesday that if America leaves the fight too rapidly, there is a risk Islamic State militants could merge with al-Qaida and plan attacks against the U.S. or its allies.

Lt. Gen. Scott Miller, a former Army Delta Force commando who heads U.S. Joint Special Operations Command, told the Senate Armed Services Committee that, if confirmed, he'll reassess the troop numbers in Afghanistan to see if the force size is right.

Any abrupt U.S. withdrawal, he said, could trigger consequences similar to what happened in Iraq in 2015. Three years after the U.S. pulled all troops out of Iraq after negotiations with the government there failed, the Iraqi military collapsed and many forces ran or surrendered to the Islamic State group as it seized swaths of territory across the country.

"A disorderly and precipitous withdrawal would have negative effects on U.S. national security," Miller said. "I would be concerned about ISIS and al-Qaida's ability to merge and project external operations — one, because I know they want to, and I know they're constantly looking for that opportunity."

Miller is expected to be easily approved by the Senate. He would take control of a war that has dragged on for 17 years and led to frustration and pointed questions about America's path to success there.

Sen. Jack Reed, D-R.I., said Miller will be taking over at a critical time. Even as the U.S. has increased troop numbers and expanded programs to train and advise the Afghans. Reed said that independent assessments find few signs of progress and the Taliban continue to launch high-profile attacks.