TRIPOLI, Libya — The branch of the Islamic State that controls Surt has expanded its territory and pushed back the militia from the neighboring city of Misurata, militia leaders acknowledged Sunday.

In the group’s latest attack, a suicide bomber killed at least four fighters on Sunday at a checkpoint west of Misurata on the coastal road to Tripoli, according to local officials and Libyan news reports.

The continued expansion inside Libya of the group, also known as ISIS or ISIL, has alarmed Western officials because of its proximity to Europe, across the Mediterranean.

Four years after the removal of Col. Muammar el-Qaddafi, the near collapse of the Libyan government has left no central authority to check the group’s advance or even partner with Western military efforts against it.