Monterrey - An unusually violent night around the northern Mexican city of Monterrey, including a string of attacks on bars, left at least 15 dead and nine wounded, authorities said Sunday.

The Nuevo Leon state prosecutor's office reported in a statement that 12 people died in six bar attacks overnight in the municipalities of Monterrey, Guadalupe and Juarez. A man and a 14-year-old boy were also shot dead in their car on a highway, and in the city of Linares, a man was killed in a clash between rival gangs after a "quinceanera" coming-of-age party at a private home.

No arrests were announced.

An official at the prosecutors' office said the death toll could rise due to the severity of some of the injuries, and it was not immediately clear if the incidents were linked. The official spoke on condition of anonymity due to security concerns.

The deadliest shooting came shortly after midnight inside the Rancho Viejo restaurant and bar on Monterrey's eastern outskirts. Five male customers and a waitress were listed as dead, and two people wounded, in an attack that the prosecutor's office said was perpetrated by two male suspects in a red SUV or pickup and was attributed to organized crime.

The Monterrey area has not seen such a deadly night in years. In 2011, 20 people were killed by a group of gunmen at a bar in a central neighborhood of the city. The following year another attack at a bar nearby killed left eight dead.