A former Guantanamo Bay inmate is leading the fight against U.S. Marines in the Helmand province of Afghanistan, a senior U.S. defense official confirmed to FOX News on Tuesday.

Mullah Zakir, also known as Abdullah Ghulam Rasoul, surrendered in Mazar-e-Sharif in Northern Afghanistan in 2001, and was transferred to Gitmo in 2006. He was released in late 2007 to Afghan custody.

Now as the United States is pushing ahead with the massive Operation Khanjar in the southern province of Afghanistan, Zakir is coordinating the Taliban fighters. Some 4,000 U.S. Marines and hundreds of Afghan forces have faced some resistance as they sweep across the province, reclaiming control of districts where Zakir and his comrades were running a shadow government.

Zakir was released from Afghan custody around 2008, according to the New York Post. He re-established connections with high-level Taliban leaders in Afghanistan and Pakistan after his second release. 

Taliban chief Mullah Omar appointed Zakir in mid-2008 as senior military commander, according to the newspaper.

Zakir quickly became a charismatic leader, helping establish an "accountability commission" to track spending and monitor activities of Taliban leaders in the districts where they held power and were running a shadow government, according to the Post.

Explaining why Zakir was released from Gitmo, the defense official said, "We were under incredible pressure from the world to release detainees at Gitmo. You just don't know what people are going to do.

"He was no worse than anyone else being held at Gunatanamo Bay," the official added. "He was not going to be tried for war crimes so we decided to release him. Either he was not thought to have committed a crime or we didn't have enough evidence to prosecute him."

The defense official shifted some blame for Zakir's activities to Afghanistan. "The country which agreed to take him promised to take steps to mitigate the threat he posed."

Afghanistan's fledgling security agencies have been having trouble keeping the country's jails secure. 

Pul-e-Charkhi prison, the country's major high-security prison that houses hundreds of Taliban and Al Qaeda fighters, has seen jailbreaks and even insurrections by inmates. Still, releases of Taliban and Al Qaeda inmates are conducted in coordination with the United States.

FOX News' Jennifer Griffin contributed to this report.