"There were 159 inmates who apparently left through the main entrance," an employee with the city's penitentiary system told AFP yesterday on condition of anonymity. The break-out from the facility in the northeastern Mexican city of Nuevo Laredo -- which lies just across the Rio Grande river from the city of Laredo in the US state of Texas -- was one of the biggest in Mexico in recent years.

The last big escape was on September 10 in northern border city Reynosa, when 85 inmates fled. Between January and September, an estimated 200 prisoners have escaped from prisons in Tamaulipas state, where Nuevo Laredo is located, according to public security officials.

The whole northern border area is the scene of a bloody struggle between two heavily armed gangs, the Gulf Cartel and Los Zetas, according to officials.

Rising lawlessness as the cartels battle it out for lucrative smuggling routes into the United States, and engaging in clashes with Mexican police and military, have raised concerns on both sides of the border.

Extra Mexican police and troops have been brought in to reinforce security at prisons. Officials said this week the number of deaths in Mexico's vicious four-year drug gang war has soared past 30,000, with 12,456 fatalities this year alone.

President Felipe Calderon launched a massive military crackdown on the cartels in December 2006, and since then there has been an escalating cycle of violence.