Mexican President Felipe Calderon has declared three days of mourning after more than 50 people were killed when suspected drug cartel gunmen set fire to a casino in northern Mexico.

Mr. Calderon condemned the violence as barbaric and said it was the worst attack on innocent civilians in Mexico in a long time. He described the attackers as terrorists who know no limits to violence.

U.S. President Barack Obama condemned the attack as “barbaric and reprehensible.”

Gunmen entered the Casino Royale in Monterrey, the capital city of Nuevo Leon, on Thursday and doused it with gasoline before lighting it on fire. Many of the victims were found inside the casino's bathrooms, where they fled to escape the gunmen but were instead trapped by smoke and fire.

President Obama on Friday pledged to continue cooperation with Mexico in the shared fight against drug trafficking. In a statement, he said the people of Mexico are engaged in a “brave fight” to disrupt violent transnational criminal organizations that threaten both the United States and Mexico.

Casinos and other businesses have been targeted by drug cartels demanding protection money.

Drug-related violence has been on the rise in Monterrey, a city of four million people once seen as a safe and prosperous. The city is about 200 kilometers south of the U.S. state of Texas.

More than 41,000 people have been killed since Mr. Calderon launched a crackdown against the country's drug cartels in late 2006.