Policemen stand outside a strip bar in downtown Monterrey August 14, 2012. Gunmen stormed a strip bar in the northern Mexican city of Monterrey late on Monday, killing eight men in a hail of bullets in an attack which bore the hallmarks of organized crime, state security and law enforcement officials said.  [Photo/Agencies]

MEXICO CITY - At least eight people were killed when six armed men burst into a bar late Monday in downtown Monterrey, capital of Mexico's northern state of Nuevo Leon, a local government spokesman said Tuesday.

The six men entered the Matehuala Bar shortly before midnight and began shooting, killing four people on the spot, including two waiters, Jorge Domene said.

The other four victims died in hospital and another was in critical condition, the spokesman added.

The assailants arrived in three vehicles, entered the bar, shot at clients and employees, then escaped, he said.

After the attack, federal police officers and military personnel arrived at the site, along with several ambulances to treat the injured.

The state's investigation agency will investigate the causes of the attack, said Domene, adding the bar is a known drug haven.

In recent days, pitched turf battles between rival drug cartels have unleashed waves of violence in Mexico, especially in the central states of San Luis Potosi and Guanajuato, the west state of Michoacan, and the northeast states of Coahuila and Nuevo Leon.

More than 50,000 people have died in Mexico from drug-related violence since President Felipe Calderon declared a war on drug in 2006.