Federal police units were sent to hunt down the gang members

Heavily armed Gulf Cartel gangsters travelling in a convoy ambushed police in the town of Cananea in Sonora state, bordering the US, on Wednesday.

They killed five policemen, sparking a battle that left 15 gang gunmen and two civilian bystanders dead.

President Felipe Calderon has ordered troops to root out drug traffickers.

This week's violence in Sonora is the bloodiest seen during a five-month offensive.

Gun supply

Up to 50 well-armed gunmen arrived in the town of Cananea on Wednesday, travelling in a fleet of rugged vehicles.

They captured and executed four lightly armed policemen in a public square.

Police teams followed the attackers to their hideout in the hills, killing 15 gunmen.

On Thursday, police helicopters and hundreds of ground forces searched for fleeing gunmen in the Sierra Madre, near Cananea.

Separately, the US and Mexico have agreed to share intelligence in an effort to try to cut off the supply of guns being used by the drug gangs.

According to the Mexican government, the vast majority of weapons used by the cartels are imported from gun shops across the US border.

Calderon priority

The continuing violence in Mexico is part of a struggle between drug traffickers and the government.

President Felipe Calderon sent more than 3,000 soldiers to Tijuana in January to fight trafficking and gang violence.

More than 300 people were killed there in 2006.

A month earlier he dispatched 7,000 troops to Michoacan state on the Pacific coast, where more than 500 people died last year.

Earlier this week two senior anti-drugs officers were shot and killed, one in the capital, Mexico City, and one in the northern border town of Tijuana.