A Mexican Defense Department press statement called it "the most important seizure of this drug in the history of the army and air force."

Authorities did not say how much heroin it would produce, but in general a kilogram of opium paste can yield about one-tenth as much of the drug.

The largest previous seizure of opiates was 245 kilograms (540 pounds) of opium paste found in Guerrero in January 2011. The Defense Department had said that seizure would have yielded over 600,000 doses of heroin.

The latest seizure was made when soldiers found dozens of large plastic containers with over 3,600 liters of the dark liquid on Feb. 1 during a raid on a drug lab in Coyuca de Catalan, a mountain town in the Pacific coast state of Guerrero, near the border with Michoacan, the department said Wednesday

The department did not explain why the seizure was not made public for over a month, but noted that in order to accurately identify the liquid, it had referred the substance to civilian prosecutors for tests.

The rugged mountains of Guerrero state, along with a few other Pacific coast states, have long been known as areas where opium poppies are grown.

But Mexican drug traffickers did not often process opium past into heroin in the past; most of the heroin trafficked through Mexico came from Colombia.

However, in recent year experts have warned that opium production and processing in Mexico may be growing.

"The availability of heroin in the United States ... is increasing as a result of increased production in Mexico," the U.S. Justice Department said in its 2011 National Drug Threat Assessment. "The level of illicit poppy cultivation in Mexico was second only to that in Afghanistan in 2009, potentially producing an estimated 50 metric tons of heroin."

On one of the major smuggling routes to the U.S. border, in Tamaulipas state, suspected drug cartel gunmen fought running battles, authorities said.

The Tamaulipas Public Safety department said gunmen first tossed a grenade into a car dealership in the state capital of Ciudad Victoria, blowing out the windows and killing one employee. Another grenade was tossed into the police academy, wounding two people, and subsequent gun battles in the city killed three other people.