Gunfights and blockades of burning vehicles have been reported in the Mexican border city of Reynosa.
The US consulate in Matamoros said in a message on Friday that it had "learned of several firefights and roadblocks throughout the city of Reynosa," and it recommended that US citizens take precautions.
The city government posted a warning on its Twitter site recommending motorists avoid several areas, including the highway to the nearby city of Matamoros.
A Mexican federal official who was not authorised to be quoted by name confirmed the burning of vehicles, roadblocks and shootouts.
But he could not immediately confirm reports a top member of the Gulf drug cartel had been detained.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Warring factions of the Gulf cartel have been fighting turf battles around Reynosa and Matamoros.
