WASHINGTON -- The U.S. Army general nominated to head U.S. military operations in Afghanistan says Washington must be careful not to withdraw too quickly, otherwise terror groups will regroup and plunge the country into deeper conflict.

In written remarks to the Senate Armed Services Committee, Lieutenant General Austin Miller declined to say what the size of the American military presence in Afghanistan should be. But he warned that an early pullout could result in Al-Qaeda and Islamic State extremists joining forces to destabilize the country.

He said an abrupt U.S. withdrawal could result in a situation similar to Iraq in 2015, when the Iraqi military collapsed after a U.S. withdrawal and Islamic State fighters seized vast swaths of the country.

Miller also said Pakistan, Russia, and Iran were continuing to support Taliban fighters.

"NATO, U.S., and Afghan efforts continue to ensure the Taliban cannot win militarily. However, military pressure alone is not sufficient to achieve a political solution to the Afghan conflict. Diplomatic and social pressure are also necessary," he said in prepared remarks published on June 19.

The Senate committee is expected to easily confirm Miller to the position.

Miller's takes over at a time when the Taliban-led insurgency continues to control large parts of the countryside and has begun a new push into major cities.

U.S. President Donald Trump last year ordered 4,000 additional troops to Afghanistan, adding to the already 10,000 U.S. military personnel already there.