Mexican security forces are scrambling to catch scores of inmates who broke out of a prison near the US border in a region plagued by drug gang violence.

A total of 151 prisoners escaped on Friday from the penitentiary in Nuevo Laredo, local public security spokesman Hector Walle said, amending earlier counts that said 141 prisoners had escaped.

Investigators have said the inmates left through a service entrance for vehicles and were helped by someone in the prison staff.

The prison warden has gone missing since the breakout, which went unnoticed until a routine head count late Thursday.

The escape from the facility in the north-eastern Mexican city - 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 entire border region has been gripped by violence and rising lawlessness as various cartels have battled over lucrative smuggling routes into the United States and clashed with Mexico's police and military.

- AFP