Here's what airline travelers can expect this summer: higher fares, long lines and jam-packed planes.

And if you wait too long to buy a ticket, you may be out of luck. With airlines loath to add flights in an uncertain economy, getting a seat at any price on some routes could be tough during peak travel times.

"I've been calling my fence-sitters and telling them they better book their flights now if they really need to go, otherwise they may be left with no options," said Jay Johnson, president of Coastline Travel Advisors in Garden Grove, Calif.

"I've been on over 20 flights within the past three months, and I would say almost every flight has been at or close to 100 percent occupancy."

The average domestic airfare this summer is up 5 percent over last summer and international flights are about 11 percent higher, according to an analysis by travel website Kayak.com. That's on top of a 15 percent average rise in airfares last year.

Getting a flight to some popular vacation destinations such as Hawaii, the Caribbean or Florida could mean facing fares that have jumped much higher than the average.

Part of the reason for higher prices is pent-up demand by Americans who cut back on travel spending during the depths of the recession and are ready to spend again, according to travel experts.

"Consumers feel more confident this year than they did last year," said Bill Miller, senior executive of travel website CheapOair.com, "and travel is the first thing people want to add back to their budget."

But even with growing demand, airlines are adding very few extra flights and routes, leery of another economic downturn or a surge in fuel prices. The merger and consolidation of several airlines in the past five years also has cut down on the number of available airline seats.

U.S. carriers filled 82.1 percent of all available seats last year — the highest annual rate in more than a decade, according to federal statistics.

"Everybody is pushing the system to capacity," said Tom Parsons, who runs the website BestFares.com. "There are less planes in the sky now."

Airlines flying near-capacity jets are also less likely to offer big discounts. This is a far cry from the recession, when air travel demand dropped steeply and airlines sold tickets at bargain prices to fill seats.

Demand started coming back after the recession, but then jet fuel prices surged nearly 30 percent in 2011. Fuel prices rose 10 percent more in the spring of this year but have been falling in the last few weeks, according to federal statistics.

If fuel prices suddenly surge again this summer, airlines will not hesitate to push fares even higher, said Jonathan Kletzel, a transportation and logistics expert with accounting firm PriceWaterhouseCoopers.

"A surge in fuel prices will impact the bottom line," he said. "And if that changes, you will see higher prices."

International fares

Fares for international flights shot up even higher this summer, on average. For example, the average cost of a round-trip ticket from San Diego to Barcelona this summer is $1,265, up 11 percent over last summer's average of $1,144, according to CheapOair.com.

London has been an especially strong draw for tourists this summer because of the queen's Diamond Jubilee celebration and the upcoming Olympics.

Because of improvements in the economy, slow to come as they are, the demand for international business travel is up.

"More Americans are going abroad but there is also more business travel from China and the Middle East," Kletzel said. "With more wealthy people flying here, that is driving the demand in international travel."