Two police officers were killed and seven others wounded in an attack on the residence of Tamaulipas state Public Safety Secretary Rafael Lomeli in Ciudad Victoria, a city in northeast Mexico, officials said Tuesday.

The gunmen threw a grenade Monday night at Lomeli's residence, which was guarded by several officers, and then detonated a car bomb as reinforcements arrived, a Tamaulipas Attorney General's Office spokesman said.

The wounded officers were taken to hospitals in Ciudad Victoria, the capital of Tamaulipas, a state that has been rocked in recent weeks by a spike in drug-related violence.

A car bomb exploded last Friday in the border city of Nuevo Laredo, located across the Rio Grande from Laredo, Texas, wounding seven people and damaging 11 vehicles.

Tamaulipas and neighboring Nuevo Leon state have been rocked by a wave of violence unleashed by drug traffickers battling for control of smuggling routes into the United States.

The army is carrying out "Operation Northeast" in Tamaulipas, Nuevo Leon, Coahuila and San Luis Potosi states in an effort to weaken the drug cartels that operate in the region.

The Gulf drug cartel and Los Zetas, a band of army special forces deserters turned hired guns and drug traffickers, are blamed for the violence.

Media reports say the Sinaloa and Gulf cartels have formed an alliance to fight Los Zetas.

Los Zetas started out as the armed wing of the Gulf cartel, but the two criminal organizations later had a falling out.

After several years on the payroll of the Gulf cartel, Los Zetas, considered Mexico's most violent criminal organization, went into the drug business on their own account and now control several lucrative territories.

Los Zetas, considered Mexico's most violent criminal organization, has been blamed for several massacres in recent years.

The cartel was accused of staging the Aug. 23, 2010, massacre of 72 migrants, the majority of them from Latin America, at a ranch outside San Fernando, a city in Tamaulipas.

Los Zetas has also been blamed for the massacre of 27 peasants in May 2011 at a ranch in Guatemala's Peten province, which borders Mexico and Belize.

Zetas gunmen set fire to the Casino Royale in Monterrey, the capital of Nuevo Leon, on Aug. 25, 2011, killing 52 gamblers and employees trapped inside, most of whom died of smoke inhalation.

President Felipe Calderon declared war on Mexico's drug cartels shortly after taking office in December 2006.

Calderon has deployed tens of thousands of soldiers and Federal Police officers across the country to combat drug cartels and other criminal organizations.

The use of the armed forces to fight drug traffickers, however, has failed to stem the violence.

Unofficial tallies put the death toll from Mexico's drug war at more than 50,000. EFE