The American air strike on a Kunduz hospital was a mistake, the top US commander in Afghanistan has acknowledged as he urged Washington to consider boosting its post-2016 military presence to repel a Taliban upsurge.

General John Campbell on Tuesday pointed to various "setbacks" in battling insurgents in Afghanistan, including those who briefly seized control of the northern city of Kunduz.

He said Afghan security forces' "uneven performance in this fighting season also underscores that their shortfalls will persist well beyond this year."

Kunduz was the site of a US air strike on a hospital that killed 22 people on October 3, an attack that Campbell described as an error.

"A hospital was mistakenly struck," he testified before the Senate Armed Services Committee, adding that "we would never intentionally target a protected medical facility."

The general stressed that while it was the Afghans who called for the strike, ultimately the decision to launch rested with Americans.

"Our forces provided close air support to Afghan forces at their request," Campbell said.
"To be clear, the decision to provide aerial fire was a US decision made within the US chain of command."

Campbell reiterated that three separate investigations were being conducted. He also said US officials were communicating with Doctors Without Borders (MSF), the medical charity that was operating in the hospital when it was struck, to get "all sides of the story."

MSF branded the strikes a war crime, and has pulled out of Kunduz in the aftermath of the attack.

Campbell said US special forces were "on the ground doing train, advise and assist" in the area near the hospital shortly before the strike.

The United States is reviewing whether to press ahead with its plan to reduce the number of its troops in the war-torn country to an embassy-based force of about 1000 beginning in 2017.