British forces racing to find the cause of a mystery fever that has struck 18 soldiers at an Afghan air base evacuated another six men to the United Kingdom for emergency treatment Thursday, a British military spokesman said.

No new cases have been reported.

``We still cannot categorically identify the source or nature of the infection,'' Lt. Col. Ben Curry told reporters at Bagram air base, about 30 miles north of Kabul.

However, he said the contagious disease resembled enteric — or intestinal — febrile illness, which he said can show meningitis-like symptoms.

British soldiers first started reporting symptoms four days ago, including fever, diarrhea and vomiting.

Two of the 18, considered seriously ill, have already been flown out of the country — one to Britain and another to a U.S. military hospital in Germany. Curry said the two were ``stabilizing.''

He said six more were evacuated by air Thursday, but were not seriously sick. Another six remain under ``barrier nursing supervision'' at the 34 Field hospital at Bagram. That means they are treated by ``medical staff wearing masks, wearing gloves, wearing aprons, all disposable, and scrupulous hygiene,'' Curry said.

While four of the 18 patients were discharged from medical care, they are still among 333 people under quarantine at the hospital to prevent the disease from spreading, Curry said.

All those under quarantine were eating sterilized military ration packs as ``a force protection measure,'' Curry said. Previously the 1,700 British troops deployed at Bagram had been eating a mixture of locally purchased food as well as military rations.

Soldiers from countries ranging from Australia to Poland are at Bagram, including about 2,700 U.S. troops. None of the U.S. soldiers was believed to have fallen ill with the disease, said U.S. military spokesman Maj. Bryan Hilferty.

``We have one American patient last night who has somewhat similar symptoms, but we don't think it's the same thing,'' Hilferty said. ``But of course we are watching everyone very carefully.''

Curry said the commanding officer of 34 Field Regiment, who was himself under quarantine, was leading the investigation at Bagram to track down the disease's cause, while doctors in Britain were examining one patient who had already arrived there.

None of the 18 soldiers with the fever had been out on field operations immediately prior to getting sick.

The area around the field hospital has been isolated and military police were deployed to keep out trespassers. British fighting forces live in green tents next to the hospital.

The hospital has treated almost exclusively British troops, though one Afghan was treated two weeks ago, officials said.

Earlier this month, three British marines were evacuated to Bagram during an operation in mountainous eastern Afghanistan. Two were diagnosed with altitude sickness and one had dysentery.

British troops at Bagram take a range of precautions to avoid illness, including using anti-malaria drugs, dipping their uniforms in insecticides and drinking bottled water.