Two of the largest online black market websites for illegal drugs have been knocked offline as part of an international operation led by the US Justice Department.

The AlphaBay and Hansa sites had operated on the dark web, a part of the internet associated with the selling of illicit items such as drugs, guns, computer hacking tools and stolen data.

FBI Acting Director Andrew McCabe said AlphaBay was ten times as large as Silk Road, one of the most widely known and controversial dark web sites, which the agency had shut down in 2013.

The US Justice Department said it had shut AlphaBay after working with international partners to close the site, which is accused of allowing thousands of vendors to sell illegal drugs.

The Justice Department said law enforcement partners in the Netherlands had also taken down Hansa Market, another dark web marketplace.

The action marked one of the largest law enforcement actions ever taken against criminals on the dark web, authorities said,striking a blow to the international drug trade that has increasingly moved online in recent years. 

AlphaBay mysteriously went offline earlier this month. It was widely considered the biggest online black market for drugs, estimated to host daily transactions totalling hundreds of thousands of dollars.

The site allowed users to sell and buy opioids, including fentanyl and heroin, contributing to a rising drug epidemic in the United States, US Attorney General Jeff Sessions said,

"The dark net is not a place to hide," Mr Sessions said at a news briefing in Washington DC to announce the action. 

"This is likely one of the most important criminal investigations of the year  taking down the largest dark net marketplace in history."

AlphaBay and Hansa Market were two of the top three criminal market places on the dark web, Europol chief Rob Wainwright said at the press conference.

The international operation to seize AlphaBay's servers also involved authorities in Thailand, Lithuania, Canada, Britain and France.

The operation included the arrest on July 5 of suspected AlphaBay founder Alexandre Cazes, a Canadian citizen arrested on behalf of the United States in Thailand.