So far, no American soldiers have reported any symptoms of exposure, U.S. Col. Roger King said.
The contamination at Khanabad air base, near the city of Karshi, was thought to be from chemical weapons stored there by the former Soviet Union, King said. Uzbekistan became independent in 1991.
U.S. forces moved into the base in October, and over the winter it was a main hub for the U.S.-led campaign in neighboring Afghanistan. At one point up to 5,000 troops were stationed there, though King said the number now is less than 1,000.
A chemical weapons team using "sniffer" devices found traces of nerve gas vapor Friday at a bunker on the edge of camp, far from any soldiers. But the next day, similar vapors were detected in a hardened hangar in which a headquarters had been set up, Maj. Gary Tallman said.
Traces of mustard gas were found Saturday in another hangar used by the Air Force for maintenance, he told reporters at Bagram Air Base, north of Kabul.
All U.S. troops have been moved away from the sites. King could not say how many had been working near the contamination or how many had been near it since October.
Investigators were trying to figure out the source of the contamination. It was not yet known how long the agents had been a hazard.
Maj. Chet Kemp, a nuclear, chemical and biological officer, said liquid chemical weapons agents may have seeped into ground and were now being turned to gas by rising summer temperatures. But he could not explain why the chemicals — if left years ago by the Soviets — would not have burned away during past summers.
Kemp and other officials did not rule out that chemical weapons were buried under the sites and were leaking. It was not known if Uzbekistan had stored chemical weapons at Khanabad since inheriting the base 300 kilometers (190 miles) south of Tashkent and 200 kilometers (120 miles) north of the Afghan border.
"There is no proof that (the contamination) was placed there recently," Kemp said. Soil samples have been sent for testing.
"The central concern now is to identify possible hazard to U.S. troops," he said. Monitors have been placed around the sites with no sign so far that the contamination had spread beyond their immediate areas.
Col. Doug Liening, chief surgeon at Bagram, said any exposure would be "low-level."
Investigators have not tested soldiers for exposure because no one has reported any symptoms. Officials were determining which units may have been near the contaminated sites "to see if there's anyone who could qualify as a potential exposure. So far our efforts have not revealed anyone who might fall into that category," Liening said.
Nerve gas interferes with the nervous system and shuts down muscle action, killing a victim who gets a strong enough dose. Initial symptoms would be "similar to the common cold," Kemp said. Depending on exposure, a victim could then show pupil dilation and suffer muscle twitching and shortness of breath.
Liening said little is known about the long-term effects of a low-level exposure. "It's reasonable to suppose that if you're not sick now, you won't be sick later," he said.
Military health teams surveyed Khanabad when troops first moved in and found nothing. They returned this week on a routine inspection and were told by Uzbek officials that Soviet troops once stored chemical weapons there, King said.
A more specialized chemical team was called in and detected vapors at the three sites.
The team will next travel to U.S. bases in Afghanistan to conduct inspections there.
Khanabad was one of several Uzbek bases used as the staging ground for the 1979 Soviet invasion of Afghanistan. The United Nations investigated claims that Soviet forces used chemical weapons against Afghan resistance fighters, but the U.N. results were inconclusive.
Russia and other former republics inherited more than 40,000 metric tons of chemical weapons from the Soviet Union. It is not known where all the chemicals were stored, especially in former republics.