The top U.S. military commander for Afghanistan told senators Thursday that he has asked for greater flexibility in how quickly he pulls troops out of Afghanistan and where he can position them around the country in the coming months.

Army Gen. John Campbell provided few details during testimony before the Senate Armed Services Committee. He said, however, that he has presented military leaders with several options that would allow him to better continue training and assisting Afghan forces, particularly through this summer's peak fighting season.

Afghanistan President Ashraf Ghani has pushed for a slower withdrawal of troops from his country, giving them more time to work with Afghan forces who will be fighting largely on their own this summer for the first time.

Current plans call for the U.S. to go from about 10,800 troops there now to 5,500 by year's end. A number of senators have made it clear they disagree with that plan, and they repeated their sharp opposition Thursday, saying the current drawdown plan is too fast and too steep.

"You've got to speak truth to power," Sen. John McCain, R-Ariz., told Campbell. "A lack of presence creates a vacuum, and we've seen what fills that vacuum in Syria and Iraq. The ungoverned spaces will allow terrorists to foment the same disaster in Afghanistan as we have seen in Iraq — growing instability, terrorist safe havens and direct threats to the United States."

By keeping more than 5,500 troops through the end of the year, Campbell would be able to maintain forces in other locations around the country, both training the Afghan forces and providing support for more counterterrorism missions.

Throughout the Iraq and Afghan wars, commanders have routinely come to Washington with requests for greater latitude in troop withdrawals, usually laying out two or three options and describing the additional security and tasks they could accomplish with each troop level.

So far, President Barack Obama has not publicly endorsed a change in the drawdown plan, but he is expected to meet with Ghani in Washington in the coming weeks.

In response to questions from Sen. Kelly Ayotte, R-N.H., Campbell acknowledged that in order to get down to 5,500 U.S. troops by the end of the year, he would probably need to begin closing facilities and moving equipment and troops during the fighting season. He said he would try to mitigate any risks and not affect the fight.

He also said he would like to have greater flexibility in positioning his troops, because the current plan would require most U.S. forces to be in the Kabul region by the end of the year.

"I'm particularly concerned about the summer of 2015, the Afghans — this is the very first fighting season completely on their own," Campbell said, adding that he has provided options that would allow the U.S. to continue the necessary training and counterterrorism operations.

Campbell also expressed concern about the potential emergence of Islamic State group fighters in Afghanistan. He said the group's possible rise is a new development and mainly represents a "rebranding" of some marginalized Taliban fighters.

"We're still taking this potential threat, with its dangerous rhetoric and ideology, very, very seriously," said Campbell. "We're working closely with the ANSF to evaluate and understand the dynamic nature of this fledgling network."

Pressed by Sen. Jeanne Shaheen, D-N.H., about the Islamic State group, Campbell said Afghans with access to media have seen a lot more about the group in the last 45-60 days.

"You do have some of the Taliban breaking off and claiming allegiance toward ISIS. Part of that is happening in different parts of Afghanistan," said Campbell. "We have seen some of the recruiting. We have seen some night letters. We have seen some talk of it at some of the universities. It is a concern to President Ghani, therefore, a concern to me."