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.

In a televised address Friday, Mr. Calderon said the United States shares part of the blame for the attack. The Mexican president said the U.S. appetite for drugs is funding huge profits for the cartels and U.S. weapons are fueling the violence.

U.S. President Barack Obama condemned the attack as “barbaric and reprehensible” and pledged to continue cooperation with Mexico in the shared fight against drug trafficking.

Gunmen entered the Casino Royale in Monterrey, the capital city of Nuevo Leon, on Thursday and doused it with gasoline before setting 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.

In President Obama's statement Friday, he said the people of Mexico are engaged in a “brave fight” to disrupt violent transnational criminal organizations that threaten both the U.S. and Mexico.

U.S. Secretary of State Hillary Clinton also condemned the attack, saying the United States stands by Mexico “now and always” as a committed partner and friend.

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 4 million people once seen as safe and prosperous. The city is about 200 kilometers south 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.