A patrol came under fire as it approached an apparent training camp spotted during an aerial search over Ciudad Mier, the Army said.

Two soldiers were wounded in the fighting, in the state of Tamaulipas.

Tamaulipas has become a major focus of violence between drug cartels competing for control of smuggling routes into the US.

The rival Zetas and Gulf cartels have been waging a bloody turf war there.

The military said three people who had been kidnapped by the gang were freed during the latest raid.

More than 28,000 people have died in drug-related violence in Mexico since President Felipe Calderon ordered the army and federal police to fight the cartels in 2006.

The rising violence has led to growing criticism of Mr Calderon's drug policy, with opposition groups saying it has done nothing to stop the flow of drugs to the US.

In his state of the union address on Thursday morning, the president admitted the violence was worsening but defended his approach, saying the cartels were being weakened.

"The capture or killing of important criminal leaders has made the crime organizations more desperate," Mr Calderon said.

"It is an ever more bloody war between organised crime groups fighting for territory, markets and routes."

Mr Calderon insisted the fight had to go on.

"If we want a safe Mexico for the Mexicans of the future, we must take on the cost of achieving it today," he said.