(CNN) -- Eleven people died Thursday in attacks in two separate towns in the northern portion of Mexico's Veracruz state, officials said.
Seven were killed in an attack on three passenger buses by a group of armed men in the city of Panuco, CNNMexico.com reported, citing a government statement.
The same group killed four people in the town of El Higo, the government said.
The buses were on the road between the towns of Tempoal and Panuco near the border of Veracruz and Tamaulipas, the government said. However, a statement from the U.S. consulate general in Matamoros said one of the buses was traveling on Highway 105 between the cities of Panuco and Tempoal.
The attacks happened early Thursday morning, the consulate said.
The Tamaulipas state highways have previously been the scene of assaults on bus passengers, according to CNNMexico.com.
The Mexican government has reported that the attacks are due to acts of extortion on bus lines and the struggle between organized crime groups. According to testimony of alleged perpetrators, the attacks are committed to prevent rival groups from entering their territories, the web site said.
The consulate general statement warned American citizens to "exercise caution and remain vigilant when traveling in the state of Veracruz. American citizens residing in, visiting or traveling through the northern portion of Veracruz that borders Tamaulipas and San Luis Potosi should maintain a heightened sense of alert." Inter-city road travel at night should be avoided, the consulate general said.
CNN's Nick Valencia contributed to this report.