Heavily armed attackers stole 20 freight containers, some carrying gold and silver, from a port in western Mexico in a heist of "unprecedented" proportions, authorities said Monday.

The burglary, described by local media as "the theft of the century," took place on June 5 in a private compound of a commercial port in the city of Manzanillo on Mexico's Pacific coast.

After incapacitating the port's security teams, the assailants used cranes and trucks to move the containers, said state security spokesman Gustavo Adrian Joya. 

"This is unprecedented. We had seen sporadic thefts of containers before, but not in such a quantity," he said, adding that the heist took eight to 10 hours.

"They were very selective in the type of goods they stole: precious metals and other things, like air conditioning units," the spokesman told reporters. 

National customs head Horacio Duarte Olivares said the area where the burglary occurred was not under the jurisdiction of the Navy, which is in charge of port surveillance.

The Public Prosecutor's Office said it was opening an investigation, but did not give details of the quantities of gold and silver stolen.