WATCH: 400 Meters From ISIS in the Battle for Benghazi

President Barack Obama recently said that failing to plan for a post-Muammar Qaddafi Libya was his biggest regret as president.

Now, nearly five years after the Libyan leader’s ouster, the North African country is ISIS’s biggest base outside of Iraq and Syria — and as tonight’s new FRONTLINE documentary reveals firsthand, both the chaos inside the country and the fight against extremists there are intensifying.

In an on-the-ground report from the war-torn city of Benghazi — where United States Ambassador Christopher Stevens and three colleagues died in an attack by Islamic militants in 2012 — journalist Feras Kilani shows just how dramatically the situation has deteriorated.

ISIS, other Islamist militants, warring militias and the Libyan army are now all fighting for command of the city. In the below excerpt from Benghazi in Crisis, Kilani embeds with one militia that calls itself the Benghazi Anti-Terrorism Unit as it battles for Sabri district, a central part of the city near the city’s port.

ISIS’s dominance in Sabri district gives it access to a supply route by sea to the other parts of coastal Libya in its grip. As Kilani notes, “If you look at the Libyan coast … at least 200 kilometers of it is under ISIS control.”

But ISIS’ presence in Sabri is just one challenge in the battle for Benghazi. Against the backdrop of a United Nations ban on arms sales to Libya, one militia member tells Kilani that they’re not equipped to fight back.

“Egypt gave us a small quantity of weapons, but it’s nothing compared to what ISIS has,” the fighter tells Kilani. “Is it because they don’t believe ISIS exists in Libya? I swear to God, look at them, they’re right in front of you.”

Watch Benghazi in Crisis for a closer look at Libya’s turbulent political situation, to see the stories of civilians displaced by the fighting, and to meet the general that some accuse of trying to become Libya’s next dictator — a charge he denies.

And keep watching for a second report from inside another country in turmoil, as journalist Safa Al Ahmad documents the Yemeni conflict’s stunning human toll in Yemen Under Siege. The special two-part report airs premieres on PBS and the FRONTLINE website starting at 10 p.m. EST. Check your local PBS station for airtimes.