BAGHDAD — ISIS militants have launched a deadly offensive against security forces in a key part of western Iraq, residents and tribal fighters said Tuesday, unleashing a wave of car bombs and seizing an area near one of the country’s largest dams.

The fresh attacks in Haditha district have set off some of the fiercest fighting in that area in months, and follow the stunning defeat of the Islamic State in the nearby city of Ramadi in December.

An Iraqi army offensive backed by U.S. airstrikes forced the jihadists to retreat from Ramadi last month, marking a devastating blow for a group striving to expand its Islamic “caliphate.”

But this week, the militants turned their guns on Haditha, a strategic area about 90 miles northwest of Ramadi and home to Iraq’s second-largest hydroelectric dam.

U.S. troops are also training Iraqi forces at the Ayn al-Asad military base just 20 miles away. The militants have had Haditha encircled for more than a year.

Troops in Haditha said the Islamic State attacks were unprecedented in the area. On Monday, the U.S.-led coalition launched two strikes on several Islamic State targets near Haditha – the first time warplanes have struck in the area since November, according to coalition statements.