Dozens of gunmen mounted rare and apparently coordinated attacks targeting two army garrisons in northern Mexico, touching off firefights that killed 18 attackers.

The attempts to blockade soldiers inside their bases - part of seven near-simultaneous attacks across two northern states - appeared to mark a serious escalation in Mexico's drug war, in which cartel gunmen attacked in unit-size forces armed with bulletproof vehicles, dozens of hand grenades and assault rifles.

While drug gunmen frequently shoot at soldiers on patrol, they seldom target army bases, and even more rarely attack in the force displayed during the confrontations Tuesday in the border states of Tamaulipas and Nuevo Leon - areas that have seen a surge of bloodshed in recent months.

The violence mainly involves a fight between the Gulf cartel and its former allies, the Zetas, a gang of hit men. The cartel - which has apparently formed an alliance with other cartels seeking to exterminate the Zetas - has been warning people in the region with a series of banners and e-mails that the conflict would get worse over the next two to three months.

Gunmen staged seven separate attacks on the army, including three blockades, General Edgar Luis Villegas said on Wednesday. In one incident, a gang of armed men opened fire from several vehicles on soldiers guarding a federal highway in General Bravo, in Nuevo Leon state.

Troops fought back, killing 18 gunmen, wounding two and detaining seven more suspects. One soldier suffered slight injuries.

Soldiers also seized 54 rifles, 61 hand grenades, rocket-propelled grenades, eight homemade explosive devices and six bulletproofed vehicles used by the attackers.

Mexico's northern states are under siege from the escalating violence involving drug gangs.

The US consulate in the northern city of Monterrey warned American citizens who may be traveling for Easter week about recent battles in the states of Nuevo Leon, Coahuila and Durango. The consulate said people traveling by road from Monterrey to Texas "should be especially vigilant."

One of the clashes between soldiers and gunmen killed two gunmen on the highway connecting Monterrey and Reynosa, which is across the border from McAllen, Texas.

Less than two hours before that shootout, Nuevo Leon Gov. Rodrigo Medina had assured citizens that authorities had regained control over the state's highways.

"I've found the highways calm. We ask that if citizens have plans to go out and enjoy these vacations, they should do so," Medina said.

Also on Wednesday, authorities in the Gulf coast state of Tabasco announced that the nephew of one of Mexico's most-wanted drug gang leaders was captured together with a police chief accused of protecting a notorious cartel in a key port city.

Meanwhile, the Mexican government announced that federal police will take over the anti-crime campaign currently headed by the army in the violent border city of Ciudad Juarez.

The army deployment has come under criticism from those who say soldiers are not trained for police work, and complaints they conducted illegal searches and detentions. But perhaps more important is the fact that killings have continued apace, even with troops in the city across the border from El Paso, Texas.

An unspecified number of soldiers will remain in Juarez to help combat drug gang violence that killed more than 2,600 people last year, and 500 more so far this year in the city of 1.3 million.