Suspected drug hitmen ambushed and killed two senior police officers with grenades and guns in Mexico, where escalating cartel violence has killed more than 4,000 people this year, police said on Monday.

In the latest of a wave of police murders, gunmen threw grenades and opened fire on the police chief in the northern state of Sonora, next to Arizona, as he walked into a hotel in the border city of Nogales on Sunday evening.

Hours later, closer to Mexico City, suspected cartel hitmen with automatic weapons shot dead the head of investigative police in the State of Mexico as he left his home early on Monday.

Twelve police have been killed in the State of Mexico since Thursday, the state attorney general's office said, and across Mexico 200 police officers and soldiers have been murdered in the past four months.

Drug gang killings and attacks on security forces have soared, leaving an unprecedented 4,060 people dead so far this year as cartels lash back at President Felipe Calderon's army-led crackdown and open new turf wars with each other.

Most victims are drug smugglers but police are frequently targeted as traffickers fighting over routes into the United States seek to instill fear in state and municipal forces or punish corrupt officers for working for rivals.

Calderon has sent some 40,000 troops and federal police to troublespots across the country but violence is only getting worse. Hitmen carried out their first major attack on the public in September with a grenade attack that killed eight civilians.

In a typical discovery, police found six bodies dumped in a warehouse in Tijuana across the border from San Diego on Monday.

In the northern border city of Ciudad Juarez across from El Paso, Texas, gunmen shot dead a 26-year-old man as he awaited surgery in a Red Cross hospital on Sunday night following a gun fight with rival drug traffickers, police said.