Mexican marines and federal agents seized 21.9 tons of ethyl phenylacetate, a synthetic-drug precursor, that had arrived at the Pacific port of Manzanillo from China, the Navy Secretariat said in a statement.

The shipment of 346 different-sized barrels from Shekou port was detected during a ship inspection, the secretariat said Thursday.

Manzanillo, Mexico's busiest port, is frequently used by organized crime gangs as an entry point for drug-precursor chemicals.

A 200-ton consignment of these chemicals was seized at that same port in 2010, a record later topped by the confiscation of an 839-ton shipment earlier this month at a warehouse in the central city of Queretaro.

Not including the seizure in Queretaro, a total of 780 tons of drug precursors have been confiscated since December 2006, when President Felipe Calderon took office and militarized the struggle against the nation's well-funded, heavily armed cartels.

Mexico, which produces most of the crystal meth consumed in the United States, has imposed tight restrictions on the import of chemicals used to make illegal synthetic drugs.