DALLAS — One of the knocks on Southwest Airlines — you'll hear it from fans of other carriers — is that you can't fly to London or Paris on one of its planes.

That won't change right away, but Southwest is finally taking baby steps into international service, including possible service to Hawai'i.

This week, it announced a deal to sell travel to Mexico in 2010 with partner Volaris, a well-financed Mexican carrier that is just 2 years old. Southwest has already said it would team with WestJet to offer U.S.-Canada travel by late 2009.

Southwest executives are overseeing a technology makeover that will modernize its reservations system to handle more international travel. They are talking to other carriers about service to Hawai'i and the Caribbean.

Competitors are paying close attention. Some may fear that Southwest Airlines Co. could emerge as a low-cost rival on their lucrative international routes, just as it pushed beyond Texas and grew into the nation's largest carrier by number of domestic passengers.

Others are courting Southwest. Last month, the chief executive of AirTran Airways said he would like to talk to Southwest about selling seats on each other's planes and sharing the revenue.

Such arrangements are called code-sharing, because one airline puts its name or code on a flight operated by the other.

Code-sharing is considered a low-risk way for airlines to expand their networks without the added cost of more planes and employees. It figures to be a particularly important strategy for Southwest, which is alone among the nation's major carriers in not belonging to one of three big global alliances or teams of airlines.

Go-it-alone Southwest's first foray into code-sharing was an afterthought — part of a move to expand at Chicago's Midway Airport in 2004. Southwest acquired six gates that had been controlled by ATA Airlines in exchange for making a cash infusion into ATA and beginning a marketing joint venture.

Southwest said it took in $50 million in revenue from the code-sharing deal in 2005. An extra benefit — the best part of the deal for some travelers — was that Southwest customers could cash in frequent-flier points for free trips on ATA to Hawai'i, which Southwest does not serve.

Soon Southwest was considering expanding the partnership to include selling seats on ATA flights to Canada, Mexico and the Caribbean. But ATA weakened and cut flights. It filed for bankruptcy and stopped flying in April. Undeterred by ATA's demise, Southwest's interest in code-sharing possibilities grew.

The timing for such agreements is also important.

Since 2001, Southwest has enjoyed fortress-like strength in the troubled U.S. airline industry, earning consistent profits because it bet right on the direction of oil prices several years ago.

But the castle walls are showing cracks.

Last month, Southwest reported its first quarterly loss since early 1991. Its wildly successful fuel-hedging bets are winding down and losing value. Its once enormous financial advantage over other airlines is shrinking.

To avoid big losses or draconian spending cuts, Southwest must raise more money — and fast.

The airline aims to increase revenue by $1.5 billion, and international code-sharing could contribute "several hundred million dollars" a year toward that goal, said CEO Gary C. Kelly.

In July, Southwest announced that by the end of 2009 it would launch service between the U.S. and Canada with WestJet Airlines Ltd. Details such as destinations, fares and revenue forecasts have not been disclosed.

Southwest officials say they have been talking to nearly a dozen airlines about code-sharing to Hawaii, Mexico and the Caribbean by late 2009 — Europe and Asia would be farther down the road.

"We want to start off regionally. It's simpler," said Richard Sweet, who leads a group at Southwest that is studying code-sharing possibilities.

"WestJet comes pretty close to the profile of an ideal code-sharing partner," Sweet said. He said both WestJet and Southwest are low-cost, efficient, and emphasize customer service.

Like Southwest, WestJet flies only Boeing 737 aircraft and its reservations system runs on the same technology from Sabre Airline Solutions, which Sweet said was a plus. And both have only one class of service — no separate first-class cabins.

Sweet said travelers will be able to buy WestJet tickets on southwest.com this year, even before the code-sharing deal starts.

Volaris differs from WestJet — and Southwest — in some ways, including its fleet, which consists entirely of Airbus jets. But it is also a low-cost carrier that hopes to capitalize on consolidation taking place in the Mexican airline industry.

Kelly said the deal opens up attractive Mexican destinations to Southwest customers. Volaris flies to 23 cities in Mexico, from border cities to beach resorts including Cancun and Puerto Vallarta.

With deals done for Canada and Mexico, Southwest will turn now to finding partners to serve Hawai'i and the Caribbean.

Sweet and Kelly declined to discuss potential partners, but industry experts all have their own favorites.

At the top of the list for several was Hawaiian Airlines Inc., to replace service lost when ATA went under.

"Hawaiian would be the obvious choice for Southwest," said Robert Mann, an independent airline consultant in Port Washington, N.Y. "In the absence of Aloha" — which failed this year — "it's the dominant carrier in the islands."

Mann said Southwest could team with AirTran or Spirit Airlines to sell seats to the Caribbean, although Spirit "is even more bare-bones of an operation than Southwest by a long stretch."

Whoever it picks, Mann said, partners will insist that Southwest begin assigning passengers to seats to match the practice of other airlines. Southwest considered such a move last year but stuck with its open-seating plan in which those who check in first get the best seats.

George Hamlin, managing director of ACA Associates, an aviation consulting firm in Northern Virginia, said Alaska Airlines could open up flights to Hawai'i and Mexico.

Some analysts said Southwest and WestJet could be the start of an alliance among low-cost carriers in the Western hemisphere, perhaps including a low-cost South American carrier such as Brazil's Gol.

"It's exciting that the LCC (low-cost carrier) sector finally is thinking about international service," said William Swelbar, a former director at Hawaiian who now runs an airline data project at MIT. "WestJet is the first step. We'll see fares to Canada come down ... that's always good for consumers."

Kelly has said Southwest wants to sell seats to Europe after 2010. Analysts said Irish discount carrier Ryanair and its U.K. rival, EasyJet, are logical partners.

But not everyone thinks Southwest will find a partner to fly its budget-conscious customers across the Atlantic.

"The last thing an international carrier wants is low-fare trash in the back of the plane," said Mike Boyd, an airline consultant in Colorado. "And Southwest doesn't need Europe."

Southwest officials are not ruling out doing some international flying with their own jets instead of relying on partners. But code-sharing is simpler — an important factor for Southwest, which likes simplicity, right down to offering only one kind of snack, peanuts.

"Partners provide branding and marketing on the other end," Southwest's Sweet said of code-sharing. "You learn about the market. It's a lot easier."