The US food safety watchdog on Wednesday warned that drinks combining caffeine and alcohol are unsafe and illegal, and ordered companies that make them to remove them from shop shelves.

"These products are adulterated and do not meet standards for safety," Food and Drug Administration (FDA) principal deputy commissioner Joshua Sharfstein told a news conference.

Companies that make and market certain very popular brands of the drinks have "failed to demonstrate that the addition of caffeine to their beverages is generally recognized as safe, and as a result, caffeine is an illegal food additive in these products," Sharfstein said.

The FDA and Federal Trade Commission sent out warning letters to four companies that make the drinks, saying if they fail to remove them from sale, they could face legal action.

FDA Commissioner Margaret Hamburg said the effort was part of a process that could lead to the products being permanently removed from the market.

"Additional steps can be taken if necessary to protect the health of the public, depending on the ability of the companies to address the concerns we have raised," she said.

The drinks targeted in the warning letters include Core High Gravity HG, Moonshot, Joose, Max and Four Loko.

According to New York Senator Charles Schumer, who has spearheaded a campaign to have alcohol-caffeine drinks banned in his state, the beverages deliver a caffeine boost equivalent to two to three cups of coffee and a dose of alcohol equivalent to three cans of beer.

"This ruling should be the nail in the coffin of these dangerous and toxic drinks," he said in a statement.

Just weeks before Wednesday's ruling, nine university students in Washington state passed out and had to be hospitalized after drinking the popular alcohol-caffeine drink Four Loko.

Washington state Attorney General Robert McKenna called the drinks "a potent and dangerous combination of huge amounts of caffeine and other stimulants with very potent alcohol.

"We didn't see these kinds of incidents when people were drinking Irish coffee or drinking regular energy drinks and adding alcohol," said McKenna.

"What we're seeing now is striking and it calls for swift action," he said, applauding the FDA's action.

State-run liquor stores in New York have stopped selling the energy-alcohol concoctions, which have also been banned in Michigan, Oklahoma, Utah and Washington state and on a number of college campuses.

Robert Brewer of federal health agency the Centers for Disease Control and Prevention (CDC) called caffeinated alcoholic drinks "a serious public health concern that can enable binge drinking, which kills 40,000 people in the United States each year."

White House drug policy director Gil Kerlikowske agreed, saying the drinks are "designed, branded, and promoted to encourage binge drinking."

Many of the drinks are sold in colorful containers that are about twice the size of a normal beer can or bottle. They also contain around twice the alcohol by volume.

The drinks are marketed to young people and have gained a sizeable following among youngsters in the past few years.

The action by the FDA follows a scientific review that began nearly a year ago, after several states attorneys general voiced concerns about the safety of the drinks.