US President Barack Obama's former spy chief has admitted that drones are causing "more damage than good" and that US prisons in Iraq "absolutely" helped in radicalising young Iraqis who later joined al-Qaeda and the Islamic State.

Retired Lt. Gen. Michael Flynn also called the US invasion of Iraq a "strategic mistake," according to reports.

Flynn quit as chief of the Pentagon's Defence Intelligence Agency in August last year. Flynn was previously a senior intelligence officer with the Joint Special Operations Command, which is behind the US military's covert and contentious drone program.

Speaking to Al Jazeera English's "Head to Head" TV show, Flynn said "there should be a different approach, absolutely" on the US military's use of drones in the war against terror.

"When you drop a bomb from a drone... you are going to cause more damage than you are going to cause good," he said.

Asked by Al Jazeera English's host Mehdi Hasan if drone strikes in countries such as Yemen and Somalia are breeding more terrorists than they kill, Flynn said he does not disagree with that view. "I think as an overarching strategy, it's a failed strategy," he said.

"What we have is this continued investment in conflict."

"The more weapons we give, the more bombs we drop, that just... fuels the conflict. Some of that has to be done but I'm looking for the other solutions," he said.

Flynn also said that the US invasion and occupation of Iraq contributed to the rise of the ISIS.

"We definitely put fuel on a fire," he said. "Absolutely... there's no doubt, I mean... history will not be kind to the decisions that were made certainly in 2003."

"Going into Iraq, definitely... it was a strategic mistake."

While denying participation in the list of abuses by the Joint Special Operations Command at Camp Nama in Iraq, Flynn pushed for greater accountability for US soldiers who took part in abuses against Iraqi prisoners.

"You know I hope that as more and more information comes out that people are held accountable... History is not going to look kind on those actions... and we will be held, we should be held, accountable for many, many years to come."

He also said that the Obama administration did not "listen" to warnings by analysts from the Defence Intelligence Agency, which foresaw "the possibility of establishing a declared or undeclared Salafist principality in Eastern Syria (...) this is exactly what the supporting powers to the opposition want" and that "the Salafists, the Muslim Brotherhood, and [Al Qaeda in Iraq] are the major forces driving the insurgency in Syria."

"I don't know if they turned a blind eye," said Flynn. "I think it was a decision, I think it was a wilful decision."