NEW YORK (CNN/Money) - Transportation stocks took a hit Tuesday, as concerns mounted that rising oil prices could drive up a key cost component of plane, truck and even train travel.

The passenger airlines were particularly hard-hit Tuesday, following a warning from Continental Airlines (CAL: down $1.05 to $26.60, Research, Estimates), the nation's fifth-largest carrier, that the rising fuel prices and continued weakness in air fares could delay its expected return to profitability next quarter.

While jet fuel prices are still far below pre-Sept. 11 levels, industry officials are concerned that a climb in prices could wipe out savings that have helped mitigate losses in the face of the decline in demand for air travel and air fares.

"When your second-highest cost plummets, that's huge," said John Heimlich, director of economic and market research for the Air Transport Association, the industry's trade group. "I call it the saving grace of the fourth quarter after the (federal assistance). But we're not quite as sanguine that we can count on that this year."

The drop in fuel prices the last six months had also been a boon to trucking companies and even their customers, as many freight carriers dropped fuel surcharges that had been in place due to higher prices. Diesel fuel prices normally see a seasonal increase in the fall as oil companies start to build inventories of home heating oil, which is essentially the same product as diesel fuel.

But this year a mild winter and weak demand for freight shipments in the fall sent prices plunging at the time they were normally climbing. However, in the last six weeks diesel pump prices are up 12 percent. The price hike is already hurting truckers' profitability, said Diego Saltes, economist with American Trucking Associations, and it could lead to a return of fuel surcharges if the prices keep climbing.

"The fact we're seeing recovery sign in economy could increase consumption and the chance of possible intervention in the Middle East is also driving prices," said Saltes. "Crude has an Iraqi premium on it right now."

Saltes said it is the smaller trucking companies without bulk purchase contracts or the clout to get customers to pay fuel surcharges who will feel the price hikes first, but that it will be felt by the public companies as well before long.

"Profitability in this industry can be very thin," he said. "Any volatility in fuel prices is devastating."

Fuel is a larger factor with the trucking companies that carry shipments that fill an entire truck trailer, known as truckload carriers.

Those carriers that handle the pallet-sized shipments of freight through a series of truck terminals, known as less-than-truckload (LTL) carriers, see relatively less immediate impact of fuel because labor and other costs make up a bigger percentage of their expense base. A series of bankruptcies in that sector, particularly among unionized carriers, in recent years has given the surviving carriers more leverage than the truckload carriers to pass on fuel surcharges when prices increase.

"Most of the LTL carriers can pretty much offset any large increase in fuel costs," said Jason Seidl, an independent transportation analyst. "It'll impact truckload more." But Seidl said a bigger concern for investors is that the trucking stocks have seen some strong gains in recent month are likely to lose some ground with new market focus on fuel costs.

"I thought some of the transport sectors ran a little ahead of themselves," he said.

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The package carriers, such as United Parcel Service (UPS: down $0.75 to $59.80, Research, Estimates) and FedEx Corp. (FDX: down $1.50 to $55.92, Research, Estimates), already have fuel surcharges in place. UPS never totally eliminated its surcharge, although it dropped from 1.25 percent last year to 0.75 percent in January to 0.5 percent currently. UPS spokeswoman Susan Rosenberg said with the current increase in fuel prices it could be heading up again in May.

FedEx reinstituted its fuel surcharge effective Monday, adding 1 percent to express shipments and 0.5 percent to shipments using FedEx Ground, its trucking network that does not use aircraft to move freight.

Of the different transport sectors, railroads have the least exposure to rising fuel costs, both because of their relative fuel efficiency and the significant other operating expenses that can place fuel below 10 percent of operating expenses. Most railroads also have long-term fuel purchase contracts known as hedges that can be used to lock in lower prices. But railroad stocks also lost ground Tuesday on fuel concerns.