MEXICO CITY, April 18 (Reuters) - An unknown assailant tossed a grenade into the offices of a northern Mexican newspaper a day after gunmen snatched a reporter from a nearby city, part of a campaign against the media by drug gangs.

The grenade exploded inside the offices of Cambio newspaper in the northern city of Hermosillo on Tuesday night, breaking windows and startling reporters. Nobody was injured, newspaper director Roberto Gutierrez told Reuters.

The attack came a day after gunmen in the city of Agua Prieta on the U.S. border, which is in the same state of Sonora as Hermosillo, snatched a crime reporter investigating the country's bloody war between rival drug cartels.

Gutierrez said Cambio had long stopped investigating drug trafficking to protect its staff but said the grenade attack was likely meant as a message to President Felipe Calderon, who is using thousands of troops to wage an attack on the cartels.

"This is a response to the federal government's decision to wage an all-out war against drug-traffickers," he said.

Mexican journalists reporting on drug gangs are often targeted by traffickers, but attacks on the media appear to be mounting as Calderon cracks down on a bloody war between cartels that has killed more than 600 people this year.

Gunmen shot dead a reporter from Mexican television network Televisa earlier this month in the Pacific resort of Acapulco, which has become a major battleground for gangs.

A vast expanse of isolated deserts, mountains and Pacific coastline that shares a long border with Arizona and part of New Mexico, Sonora state is a strategic transit point for South American cocaine bound for the United States.

More than 2,000 people were killed last year as Mexico's notorious Gulf Cartel battled over production, routes and markets with an alliance of traffickers from the state of Sinaloa, which borders Sonora to the south.

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