Mexico City, Mexico (CNN) -- At least nine people died in shootouts over the weekend in northeastern Mexico, an area where two drug cartels have been waging a bloody war since January.
Eight people were killed early Sunday in the municipality of Miguel Aleman in Tamaulipas state, the state government said on its Web site. Five died inside a bar and three others were killed in a motor vehicle, the government said.
One gang-related death was recorded in the municipality Saturday during a firefight between soldiers and a group of armed men, the government's information center said.
The area, which borders Texas, has seen ferocious fighting between the Zetas and Gulf cartels after a recent gangland slaying. Hours-long gun battles are common, and U.S. officials were recently forced to temporarily close the consulate in Reynosa, another city in Tamaulipas state.
On Monday, the U.S. State Department issued a travel warning to inform U.S. citizens traveling to and living in Mexico of concerns about the security situation in the nation. The State Department issued a similar warning in March.
The trouble in Tamaulipas and neighboring Nuevo Leon state started January 18, when Gulf cartel members killed top Zeta lieutenant Victor Mendoza. The Zetas demanded that the Gulf cartel turn over the killers, but the narco group refused.
The Zetas, composed mostly of former elite military troops, had been the armed enforcers for the Gulf cartel since 2001. They have become more independent in recent years, and the all-out war between the two cartels indicates the split is apparently permanent.
Mexican authorities said Monday that the Gulf cartel has reached an alliance with the Familia Michoacana drug organization to fight the Zetas.
Federal police made the observation in an announcement about the arrest of a top Familia Michoacana lieutenant and two other suspects.
Cartel suspect Jose Manuel Cuevas Arias, known as "el Borrego," was the Michoacan organization's leader in the state of Guanajuato, the federal police agency said on its Web site.
The other two suspects were identified as Rafael Rosales Cuevas, 32, alias "el Aguacate," and Luis Alberto Farias Ruiz, 22, known as "la Lore."
The states of Michoacan and Guanajuato border each other in southern Mexico.
Cuevas Arias, 31, is accused in the deaths of four federal police officers, the agency said.
He was incarcerated twice in the United States, sentenced in Moreno Valle, California, in 2000 for the production of amphetamine and later for illegal entry into the country, Mexican federal police said. He was sentenced to two years for the illegal entry and was released in 2007, the federal police said.
The weekend violence in Tamaulipas was the latest in a string of bloody killings in the state. Earlier this month, five men and two women were killed in the city of Tampico.
The most violent day this year occurred March 30, when 18 people died in drug-related violence, the state government said.
No official figures are available, but media tallies this year say more than 17,000 people have been killed nationwide since Mexican President Felipe Calderon declared war on the drug cartels shortly after taking office in December 2006.
Ciudad Juarez, in northern Mexico across from El Paso, Texas, is the most violent city in the nation, with more than 2,600 drug-related deaths in 2009. No official numbers are available, but more than 400 killings have been reported by local media this year.
The city, long the focal point of Calderon's battle against cartels, came to renewed prominence after the January 31 killings of 15 people, most of them students with no links to organized crime. The massacre sparked outrage throughout Mexico and drew worldwide attention.
In another incident that drew widespread attention, three people associated with the U.S. Consulate in Juarez were gunned down in two shootings last month. Two of the victims, including a pregnant woman, were U.S. citizens living in El Paso, Texas.
Two graduate students apparently killed in crossfire between Mexican military and drug gang members in Nuevo Leon state also garnered much attention last month. Nuevo Leon borders Tamaulipas and also has seen widespread violence because of the battle between the Gulf and Zetas cartels.
The Mexican government apologized to the families of the two students and promised an investigation into how they were killed.