The U.S. Consulate in Matamoros warned on Wednesday that gun battles between Mexican government forces and gunmen on roads between that border city and Reynosa were likely to continue.
The death toll from fighting in the area stood at nine.
The consulate had warned on Tuesday of "rolling gun battles" in the area in northern Tamaulipas state and advised its employees to stay off the streets.
On Wednesday, it distributed an emergency message for U.S. citizens, warning of a "likelihood of increased violence in the Matamoros vicinity, reportedly between the Matamoros and Reynosa factions of the Gulf cartel."
The warning noted numerous reports of large convoys of armed members from what it called transnational criminal organizations, or TCOs, driving through Matamoros.
Matamoros and Reynosa are dominated by the Gulf drug cartel, which has seen internal divisions and turf battles with the rival Zetas gang.
Gang members had apparently been preparing for a battle by blocking roads around the two cities.
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