Twelve U.S. Marines who were in Liberia (search) last month have been diagnosed with malaria (search) and 21 other U.S. troops have symptoms of the disease, defense officials said Monday.

Two of the Marines were flown from the USS Iwo Jima (search) warship off the coast of Liberia to Landstuhl Regional Medical Center in Germany on Saturday. Thirty other Marines, plus one sailor, were flown Sunday to the National Naval Medical Center in Bethesda, Md., according to a brief statement from U.S. European Command, which is in charge of the Liberia mission.

A spokesman at the Bethesda center, Lt. Cmdr. Chito Peppler, said he could not comment on the patients' condition.

Col. Jay DeFrank, a Defense Department spokesman, said the Marines, members of the 26th Marine Expeditionary Unit based at Camp Lejeune, N.C., were in Liberia in mid-August as part of a U.S. quick-reaction force of about 150 U.S. troops. They operated from an airport outside Monrovia, the capital.

In addition to the 12 confirmed cases of malaria, test results on the 21 other patients are pending, Peppler said.

U.S. troops normally receive an anti-malarial drug regimen before deploying to a country like Liberia where there is risk of getting the disease. Peppler and DeFrank said it was not immediately clear whether the Marines who fell ill had taken such medication.

The European Command statement said all 33 patients were responding to treatments, but it provided no other details.

The two who were flown to Landstuhl exhibited more severe symptoms than the 31 others, officials said.

European Command said it was unlikely that the illness presented a communicable disease risk to the other Marines and sailors aboard the ships.

Other officials said that as a precaution, movements of Marines ashore in Liberia from the Iwo Jima and two other U.S. Navy ships off the Liberian coast were being limited for the time being.

There are about 136 U.S. troops ashore in Liberia, mostly Marines providing security at the U.S. Embassy in Monrovia.

The Iwo Jima and two other ships off the coast have about 2,200 Marines and about 2,500 sailors aboard.

Malaria is transmitted by mosquitos that breed in stagnant water and tall grass.

The disease kills 3,000 children a day in Africa and robs the continent of millions of dollars in lost productivity, the United Nations said in a report early this year.

The mosquito-borne disease infects 300 million people a year in the poorest continent and has become increasingly resistant to drugs, said the World Health Organization and the United Nations Children's Fund, UNICEF. Still, they said, there are ways to control the disease.