U.S. authorities have seized 51 packages of counterfeit Tamiflu, a drug that can be used to treat potentially deadly Avian flu, a customs service spokeswoman said on Monday.

Agents seized the shipments at a U.S. airmail facility in South San Francisco in an operation that began in late November and anticipate more coming into the United States by mail, said Roxanne Hercules, a spokeswoman for the U.S. Customs and Border Protection service.

“It’s our expectation that we would probably see more,” said Hercules, noting the substances seized by federal agents had only trace elements of Tamiflu.

The substances had been ordered over the Internet and shipped from Asia, Hercules said, but gave no more details.

Tamiflu, manufactured by Roche, is being stockpiled worldwide amid fears of a flu pandemic. Doctors believe the drug would help to control H5N1 avian influenza, more widely known as bird flu.