Drug traffickers fighting to control northern Mexico have turned their guns and grenades on the Mexican army, authorities said, in an apparent escalation of warfare that played out across multiple cities in two border states.

In coordinated attacks, gunmen in armored cars and equipped with grenade launchers fought army troops this week and attempted to trap some of them in two military bases by cutting off access and blocking highways, a new tactic by Mexico’s organized criminals.

In taking such aggressive action, the traffickers have shown that they are not reluctant to challenge the army head-on and that they possess good intelligence on where the army is, how it moves and when it operates.

At least 18 alleged attackers were killed and one soldier wounded in the fighting that erupted Tuesday in half a dozen towns and cities in the states of Tamaulipas and Nuevo Leon, the army said, topping off one of the deadliest months yet in a drug war that has raged for nearly 3 1/2 years.

The U.S. Consulate in Monterrey issued a warning to Americans who might be traveling in northern Mexico for the Easter break, citing the sudden outbreaks of gun battles in Nuevo Leon and neighboring states.

Traffickers previously have fought with army patrols, but the attempt to blockade garrisons came after weeks of an intense, bloody power struggle between two rival organizations, the Gulf cartel and its erstwhile paramilitary allies, the Zetas, to control the region bordering South Texas.

Part of the strategy of Tuesday’s assaults may have been to prevent the army from patrolling, to give the drug gangs a freer hand in their fight against each other.

“This really speaks to the incredible organization and firepower that the drug-trafficking organizations have managed to muster,” said Tony Payan, a border expert at the University of Texas at El Paso. “These are organizations that are flexible, supple and quick to react and adapt. They no doubt represent a challenge to the Mexican state.”

In Reynosa, one of the scenes of Tuesday’s fighting, the local government put out alerts Thursday for residents to avoid parts of the city. Residents said they heard gunfire and saw military armored personnel carriers moving through neighborhoods. One person was reported killed.

“People hear gunfire and get scared,” said Jaime Aguirre, a radio talk show host. “But it’s better to keep quiet and not hear anything so as not to risk reprisals.”

Reynosa resident Yenni Gandiaga was driving to the gas station Tuesday morning when she heard gunfire getting closer and louder. Then she saw the troops and the gunmen. She turned down a side street to hide, crashing into two other cars in the process.

“People ran about screaming, picking up their children,” she said. She hid in a stranger’s house. When she emerged after the combatants moved on, the windows of storefronts and cars were shattered.

The Mexican Defense Ministry in Mexico City put out a blow-by-blow account of Tuesday’s events. Taking a page from a manual on urban guerrilla warfare, gunmen struck at the same time Tuesday morning, and then again in the afternoon.

In Reynosa, a city in Tamaulipas state across the border from McAllen, Texas, gunmen positioned trucks, cars and trailers on a highway to block Campo Militar, an army base, about 11 a.m. At almost the same time, they blocked a garrison in the city of Matamoros, about 60 miles to the east. In Rio Bravo, between the two cities, traffickers battled with army patrols.

Later in the day, troops and traffickers clashed in other Tamaulipas towns and in neighboring Nuevo Leon state.

The army said it confiscated armored cars, grenade launchers, about 100 military-grade grenades, explosive devices and about 13,000 rounds of ammunition. Seven men were captured.

“The actions by these criminal organizations are a desperate reaction to the advances made by federal authorities in coordination with state and municipal security forces,” Gen. Edgar Luis Villegas said.

It was not clear whether the fighting the army reported was with the Zetas or the stronger Gulf cartel. Most of the violence has been cartel against cartel, with some bystanders getting caught in the cross-fire. The gangs have also attacked police stations in many areas.

The Zetas, founded as a group of mercenary former soldiers working for the Gulf group, split away in a bid to take over part of the lucrative drug trade. They are fighting to seize territory from the Gulf network in Tamaulipas and Nuevo Leon, amid reports that other strong cartels, such as the one based in Sinaloa, may be uniting with the Gulf traffickers to wipe out the Zetas.

Dozens of people, primarily traffickers, have been killed in recent weeks as the two groups clashed in the broad triangle along the border from Nuevo Laredo to Monterrey to Reynosa and Matamoros. Traffickers have flexed their muscle by repeatedly setting up roadblocks, closing highways and tying up traffic even in Monterrey, a major city.

“It is a risky tactic because it has the potential of angering society, but it is a very effective show of power,” said Martin Barron, a researcher at a Mexico City think tank.

The increased agility of the drug gangs seen in Tuesday’s violence indicates good intelligence, experts here and abroad said. Some of that intelligence comes from taxi drivers, street vendors and scores of other people on the traffickers’ payroll who serve as lookouts for drug runners and their henchmen. But Payan and others suggested that some of the precise, street-level intelligence may come from soldiers, adding substance to fear that as the army is increasingly dragged into the drug war it is becoming susceptible to the same cartel-financed corruption that has long corroded police departments and many political structures.

In Ciudad Juarez, Mexico’s deadliest city and where the army has been deployed in greatest force, federal police are to begin taking over security duties this month as the army is gradually withdrawn, the government said. The army has been criticized for rights abuses, including the disappearance of detainees and illegal searches.

By one newspaper’s count, the drug war’s death toll in March was the highest yet, more than 1,000.

wilkinson@latimes.com