'Very Severe Collusion'

In Nuevo Laredo, soldiers and federal police surrounded the prison. Tamaulipas' security chief Antonio Garza told local radio that the jail's director was reported as missing along with 141 inmates. He confirmed the vehicle entrance was used as the escape route and blamed prison guards for complicity.

"I am sure that inside (the prison) there was very severe collusion (between guards and drug gangs)," he said. The head of the state's jail system was also suspended pending investigation, Garza added.

It was not immediately clear who was behind the prison break, but police sources said the Gulf cartel may have offered to free Zeta gang members, their former allies, on condition that they switch sides.

The escape follows a string of breaches across northern Mexico, underscoring the challenges that President Felipe Calderon faces as he battles powerful drug cartels.

Authorities have arrested or killed at least seven top drug cartel leaders in the last year, but have not succeeded in shutting down the gangs.

Calderon, who sent thousands of troops across the country to fight drug gangs, has vowed to clean up prisons that in the past have allowed drug lords to live in luxury or escape when they please. But the conservative leader has struggled to contain corruption and lawlessness in the prison system, which is run partly by state and local governments.

In September, 85 prisoners escaped from a prison in the nearby border city of Reynosa. Authorities discovered in July that prison officials had allowed convicts out of a prison in northwestern Durango state to carry out revenge attacks before returning to cells for the night.