Taco Bell is pulling green onions from its 5,800 restaurants in the United States after early tests showed they may have caused an E. coli outbreak that sickened at least three dozen people.

Independent labs found that three samples of green onions carried a strain of the E. coli bacteria. Taco Bell officials noted the tests are not yet conclusive.

"In an abundance of caution, we've decided to pull all green onions from our restaurants until we know conclusively whether they are the cause of the E. coli outbreak," said Greg Creed, president of Taco Bell.

Taco Bell restaurants in Canada use different food supply chains and have not affected by the E. coli outbreak, officials said Wednesday.

The fast-food chain on Tuesday reopened its Long Island, N.Y., restaurants but closed nine of its outlets in Philadelphia after four reports of E. coli illness were reported in the area. Nine people remain in hospital in New York and New Jersey, including an 11-year-old boy suffering from kidney damage. The most recent case of E. coli was reported on Nov. 29.

State health officials say they believe the danger of infection has passed, but are continuing to inspect restaurants and plant distribution centres.

E. coli stands for Escherichia coli, a bacterium commonly found in the intestines of animals and humans. The bacteria can be found in undercooked meat, on sprouts and leafy vegetables and can cause abdominal cramps, fever, bloody diarrhea, kidney failure, paralysis and death.

Earlier this year, an E. coli outbreak linked to bagged spinach sickened more than 200 and killed three people.

With files from the Associated Press