The brigades have advanced more swiftly than many expected on the Islamic State's stronghold in Libya, though their progress has been hampered by suicide bombers, mines and snipers.
If the advances are sustained, they could dislodge Islamic State from its most important base outside the Middle East and provide a boost to the UN-backed Government of National Accord.
Muammar Qaddafi is gone and his sons are either dead or in prison, but Libya is still convulsing as rival governments vie for control of the oil-rich nation.
Libya named a new national unity government on Tuesday — a small but important step toward ending the nation's bloody, multi-party civil war. Looming over the ordeal is the specter of the Islamic State, which is expanding a mini-state in Sirte.
In the years since Qaddafi's fall, Libya has been torn apart by political instability, militia violence, and Islamist extremist groups.
The Islamic State released a video that appears to show a pair of executions in Libya, including one that involves a man being dragged to death behind a truck.
Reports of beheadings and bodies strung up on poles have emerged following an attempted rebellion against Islamic State militants in the Libyan city of Sirte.
A massive, peaceful protest in Addis Ababa over the Islamic State’s murder of more than 20 Ethiopian Christians turned violent as demonstrators called for vengeance.