WASHINGTON - The Food and Drug Administration is stepping up its fight against counterfeit and other potentially harmful medicine sold over the Internet.

The agency is ordering operators of about 4,100 websites to immediately stop selling unapproved medications to U.S. consumers.

The FDA has sent warning letters to three companies behind most of the websites. It's also seized some illegal medicines, filed civil and criminal charges against companies and people, and contacted Internet registrars and service providers, asking them to suspend the 4,100 websites.

Those efforts are part of a global crackdown on online sellers of counterfeit and illegal medical products. Interpol, the international police agency, says about $10.5 million worth of medicines were seized worldwide last week and more than 18,000 illegal pharmacy websites were shuttered.