Soldiers and police in El Salvador have surrounded the entire central Cabanas department as part of President Nayib Bukele's ongoing war against gangs.
"Since this morning, 7,000 soldiers and 1,000 police officers have established a security fence," Mr Bukele posted.
The aim is to stop gang members leaving and to break their supply chains.
More than 70,000 suspected members have been arrested since March 2022, when a state of emergency was declared after a spike in gang-related murders.
The impoverished Central American nation is seen as one of the most dangerous countries in the world.
Thousands of people with no discernible link to gang activity have also been swept up in the dragnet of arrests.
There are also concerns about a recent move by the country's lawmakers to allow mass trials.
In a series of posts on X, formerly known as Twitter, President Bukele said Cabanas had "become the place with the largest number of terrorist cells", where gang members were hiding in rural areas.
He stressed that the siege would not be lifted until "all the criminals" were apprehended.
At the same time, the president urged local residents and tourists to stay calm and carry out their normal activities.
Lorries loaded with soldiers were seen on the streets of the regional towns of Tejutepeque and Ilobaso on Tuesday, AFP news agency reports.
Cabanas is an agricultural region that is home to more than 160,000 people.
It covers an area of just over 1,000 sq km (390 sq miles).