RAF LAKENHEATH, England — U.S. Air Force officials warned Thursday that their biggest fighter wing in Europe could suffer long-term damage if Iceland's volcano keeps belching ash into the skies.

The Air Force sent two F-15 fighters on test flights Wednesday and another eight on Thursday, but said not enough data has been gathered to resume normal operations, despite the lifting of the civilian flight ban and the return to the skies of commercial airliners.

Col. John Quintas, an F-15 pilot who commands the 48th Operations Group, said the test flights all returned safely after about 75 minutes in the air. The fighters did not encounter any major problems, though some pilots reported haze.

"They certainly didn't come down clean, but they did come down safe for flight," he said.

He said mechanics were analyzing the fighter engines to determine the impact of the ash, but added that the risks might not be immediate.

"What we are concerned about is the long-term effects of low-level exposure," he told the Associated Press at RAF Lakenheath, an Air Force base about 80 miles northeast of London. Lakenheath's 82 F-15s make it the largest U.S. fighter base in Europe.

Normally, about 50 to 60 fighters a day take off on missions from the base. On Thursday, only the eight test flights were allowed.

"We are in uncharted territory," Quintas said. "We could be in this situation for months."

The safety of the military fighters has been watched as an indicator of whether commercial airliners should fly and the first forays were not good — test flights by a Belgian F-16 and two Finnish F-18s over the weekend, before the civilian ban was lifted, resulted in minor engine damage.

But Quintas said the two kinds of planes are not easily compared.

Fighters are potentially more susceptible to the ash than commercial airliners because their engines operate at higher temperatures and under greater pressure. The heat from the jets can turn the ash into glass, clogging up the blades and causing the fighter to lose power.

Another problem for the fighters is that they are likely to be flying in the same skies over a longer period of time, he said, so could be more deeply affected than airliners by long-term accumulation of the ash in their flight systems.

"We are being cautious because we have a lot of unknowns that we are working our way through," said Col. Dorothy Silvanic, the 48th Maintenance Group commander. "The jury is still out. We are not sure yet what we are seeing."

Silvanic also said the big question mark is what happens if the volcano does not quiet down.

"Our main concern is the potential for cumulative damage," she said. "An aging fleet needs lots of care and feeding — and we have an aging fleet."

The F-15 has been in service since the 1970s.

Even so, Quintas said that the ash from the volcano appeared to be leveling off at a lower altitude than after the initial eruption — making flying above it safer — and that weather patterns were changing.

"I don't think we're in danger of having to close the airfield," he said. "But when we look at whether we should be flying the airplanes in this environment, it's not going to do me any good to trash the engines."