Islamic hard-line militias, including the group accused by the United States in a 2012 attack that killed the ambassador and three other Americans, claimed control of Libya’s second largest city, Benghazi, after overrunning army barracks and seizing heavy weapons.

The sweep in the eastern city is part of a new backlash by hard-liners against their rivals ahead of the sitting of a new parliament. In the capital Tripoli, escalating battles Thursday between militias prompted multiple foreign governments to scramble to get out their citizens as thousands of Libyans fled across the border into Tunisia.

The weeks-long surge of violence renewed fears that Libya, which has been in chaos since the 2011 civil war that ousted longtime dictator Moammar Gadhafi, is plunging deeper into civil strife. […]