Zuwara is one of the points along Libya's western coastline where smugglers and traffickers hold migrants before putting them on boats to try to cross to Europe.
The migrants were from various sub-Saharan African and Arab countries, as well as Pakistan and Bangladesh, the statement said. They had been shut inside a refrigerated container designed for transporting meat or fish which was found just outside Zuwara, close to the Mellitah oil and gas complex.
Zuwara is about 110km (68 miles) from the capital, Tripoli.
"As a result of the length of time they were suffocated, eight of them died including six children, one woman and a young man," the security directorate said, adding that gallons of petrol had been found in the container.
It posted pictures of at least nine plastic jerry cans inside the container, as well as a pile of life-jackets apparently intended for use in a boat crossing.
Daytime temperatures in northwest Libya have been in the mid to high 30s Celsius in recent days.
Smugglers and traffickers took advantage of Libya's lawlessness to send hundreds of thousands of migrants to Italy over the past four years, though flows have slowed since last summer due to an Italian-backed crackdown on smuggling networks.
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