Venezuela is waging its most concerted military campaign in years, targeting what it says is a criminal group operating within its border near Colombia but also sending an estimated 5,000 of its own civilians fleeing into the neighboring country.

The assault, which began with several days of airstrikes that security experts described as Venezuela’s largest use of firepower in decades represents a significant departure from the largely hands-off approach it has long employed toward the illicit organizations that flourish along its border.

For years, officials in President Nicolas Maduro’s government have tolerated and sometimes even cooperated with these armed groups, many of them with roots in Colombia, as they moved drugs and other contraband between nations.

Now it has lashed out at one of them, though the reasons remain murky. Mr. Maduro has claimed in recent days that the attack reflects his government’s policy of “zero tolerance toward irregular Colombian armed groups.”

The Venezuelan assault, centered around La Victoria, a town of about 10,000, has been aimed at a faction of FARC dissidents known as the Tenth Front, according to local residents, leading security experts to suggest they may have broken unwritten rules laid out by the Maduro government or its allies.