MEXICO CITY, MEXICO - Five bodies, one without a head, were found in a car in the northern Mexican city of Monterrey, where a separate shootout left three dead at the weekend, justice officials said on Monday.

A shootout outside a baseball stadium meanwhile interrupted a league game in the border city of Reynosa, and more than 30 people were killed in Ciudad Juarez over the weekend, including two US citizens, officials said.

Almost 23,000 people have died in spiraling drug-related attacks in Mexico the past three and a half years, despite a military clampdown on organized crime involving some 50,000 troops.

Police found five bodies in a car in the south of Monterrey overnight Sunday, an official from the Nuevo Leon state prosecutor's office said, declining to be named.

One victim had been decapitated - a common practice among warring drug gangs - and the bodies were accompanied by a message threatening supporters of "an organized crime group," the official said, without naming the gang.

Violence has surged in the previously calm city of Monterrey in recent months as the powerful Gulf cartel battles its former allies the Zetas across northern Mexico.

Three suspected criminals died in a daylight shootout with police in Monterrey's city center on Saturday, the official added.

Further north, a shootout outside the Adolfo Lopez stadium in the city of Reynosa, across from McAllen, Texas, forced baseball players and referees to abandon the pitch on Sunday, local reporters said.

No injuries were reported, but the match, between the Reynosa Broncos and the Monterrey Sultans, was suspended and rescheduled for Friday, said Gabriel Medina, chief media officer for the Mexican Baseball League.

Local officials meanwhile reported more than 40 killings over the weekend in the northern state of Chihuahua, including 32 in Mexico's murder capital of Ciudad Juarez, a hub for drug trafficking routes into the United States.

Two US citizens of Mexican origin were among the dead, killed Saturday inside their car in Ciudad Juarez, across from El Paso, Texas. A passenger was also wounded, local justice officials said.

A cameraman from the local offices of the State Commission of Human Rights was also shot dead in his car Saturday, in the state capital Chihuahua, rights groups reported. --AFP