Medications produced at a US GlaxoSmithKline pharmaceutical plant that was closed when the bacteria that causes potentially fatal Legionnaires' disease was discovered are safe, the company has said.

The London-based drug maker expects to reopen the manufacturing plant in Zebulon, about 25 miles east of Raleigh, North Carolina, which makes inhaled medications, within three days.

The plant was closed on Tuesday after routine testing found legionella bacteria in two external cooling towers.

"No employees are sick and no products have been compromised," spokeswoman Jenni Ligday said. "Medicines were not exposed to the bacteria."

North Carolina health chiefs said no cases of Legionnaires' disease linked to the Glaxo plant had been reported.

People can contract Legionnaires' disease when they inhale water vapour or mist containing the bacteria, the Centres for Disease Control and Prevention said. It does not spread from person to person.

GSK said the company shut down production and closed the building because the cooling towers maintain temperature and humidity needed for manufacturing and working conditions. None of the air or water released by the cooling towers "come directly into the building or in contact with any of the products", Ms Ligday said.

A US Food and Drug Administration spokesman said Wednesday the drug safety regulator was still looking into the details and referred questions to GlaxoSmithKline.

About 600 of the 850 people GSK employs in Zebulon were affected by the building closure.

The plant produces inhaled drugs like Advair, a drug for asthma, and contracts with other pharmaceutical companies to produce their drugs.

Legionella bacteria found in cooling tanks in the Bronx section of New York City caused 12 Legionnaires' disease deaths this summer.

Most people who are exposed to the bacteria do not become ill, but about 8,000 to 18,000 Americans are taken to hospital with the disease every year, the CDC said.