E-mail inboxes may be clogged with a little less spam — at least for a while. U.S. authorities said Tuesday they have shut down one of the largest spam operations in the world, a vast network involving countries from New Zealand to China and the United States.

The spammers sent out billions of e-mails in recent years encouraging people to click through to websites that allegedly used false claims to peddle prescription drugs, as well as "male enhancement" and weight-loss pills.

The U.S. Federal Trade Commission received more than three million complaints about the spam and related websites, illustrating the scale of the operation, officials said.

The sites, including one called "Canadian Healthcare," were difficult to distinguish from legitimate online pharmacies — making the pitches more persuasive, said Steve Baker, the FTC's Midwest Region director.

"These sites are really professionally constructed," he said. "Some years ago you used to be able to tell the bogus things because they looked cheesy and had misspellings. Anymore, I don't think that's true."

The operation violated the U.S. federal CAN-SPAM Act of 2003, meant to restrict commercial spam, by using false header information to hide the origin of messages, not offering an opt-out link and failing to list a postal address, the FTC said.

As part of their inquiry, FTC staff made undercover purchases from the sites. No one asked the clandestine buyers to provide verification of a prescription and the shipped drugs did not include doctors' instructions or dosage information, officials said.

A federal judge in Chicago issued a temporary injunction to halt the operation and also froze its assets. The Federal Bureau of Investigation is investigating and those involved could also face criminal charges, Baker said.

Those spearheading the enterprise, known as "Affking" on the internet, included a U.S. and a New Zealand citizen, according to court documents.

Servers in China hosted the websites and the drugs were shipped from India, while operatives in Cyprus and the former Soviet republic of Georgia processed credit card information, Baker said.

The case should remind consumers to beware of spam, he said.

"If you find your way to these websites through spam, you should really be asking yourself if you can trust them at all," Baker said.