Internet operators selling prescription drugs on the web have been warned that they may be acting illegally if they sell to US citizens.

US authorities are concerned that patients may be at risk from rogue traders peddling fraudulent, unapproved or inappropriate products.

The US Food and Drug Administration (FDA) has written via the internet to a dozen operators of websites outside the US warning them they could be breaking the law.

The FDA said the sites, which it did not name, may have been offering medicines to US citizens without a prescription or otherwise violating US laws.

It is the first time the FDA - which is determined to crackdown on unregulated online pharmacies - has issued warnings of this sort on the internet.

In a statement, the FDA said the letters "represent a new stage in the agency's efforts to protect the public against illegal and potentially dangerous products sold through websites".

Shipments could be detained

The letters, which were also sent on paper, warn the companies that future shipments may be detained from entering the United States.

"These drugs could pose a serious risk to patients, particularly if used without proper medical supervision," the FDA said.

One recipient has already told the FDA it would cease illegal activities.

Federal regulators and lawmakers are worried about possible health risks posed by some of the hundreds of online pharmacies.

Some offer medicines, including popular drugs such as the anti-impotence pill Viagra and weight-loss remedies, without a valid prescription.

Others may be offering fraudulent or unapproved products.

Officials say the websites are difficult to track.

The Clinton administration has proposed bolstering FDA authority and manpower to investigate the sites and imposing fines.

Also, it wants the power to certify pharmaceutical websites as legitimate.

Richard Ley, a spokesman for the Association of the British Pharmaceutical Industry, said patients using online pharmacy services should be very cautious.

He said there were question marks about the way online pharmacies carried out checks to ensure that a patient had received proper medical advice before dispensing prescription drugs.

"Also, one has to question whether patients are really getting what the website says they are getting. Are the drugs within their sell-by date, and have they been stored in the correct conditions?"

It has been predicted that the domestic US market for online health services will increase from $200m in 1999 to $10bn in 2004.