LAS VEGAS — Despite the weak U.S. dollar, a boom in international travel around the world hasn't translated into an explosion of foreign tourists to the United States.

Explanations range from post-9/11 security headaches and lower airfares elsewhere to poor marketing by the U.S. Whatever the cause, travel industry experts say the U.S. is missing an opportunity to make up for the shortfall in domestic tourism caused by high fuel prices.

At Heli USA Airways, one of several operators that whisk visitors on aerial tours of the Las Vegas Strip and nearby Grand Canyon, vice president of marketing and sales John Power said the faltering U.S. economy and competition from other countries are crimping business.

"Right now, there's some other worldwide destinations that are taking some of the marketplace," Power said.

According to the U.N. World Tourism Organization, the United States had 51 million international visitors in 2000, more than 7 percent of the 682 million international arrivals worldwide. But as international arrivals worldwide jumped to 846 million in 2006, the U.S. saw roughly the same number of visitors as it used to — dropping its share to 6 percent.

The U.S. share of international tourism dollars has slipped, too, though the U.S. still drew more money than any other single country in 2006 and more than it did in 2000. From 16 percent of the market in 2000, or $82.4 billion, the U.S. took in 12 percent of the $733 billion worldwide tourism market, or $86 billion in 2006.

Major destinations such as Los Angeles, Orlando, San Francisco, Miami, Honolulu, Las Vegas, Chicago, Washington, D.C. and Boston all saw 20 percent to 34 percent fewer travelers in 2006 compared with 2000. Of the top 10 cities, only New York saw more visitors in 2006 than in 2000, with a 9 percent increase to 6.2 million arrivals, according to the U.S. Commerce Department.

Nearly 26 million people traveled to the United States from overseas in 2000. But that dropped drastically after Sept. 11, according to data from the U.S. Commerce Department's Office of Travel & Tourism Industries. The number bottomed out in 2003 with 18 million overseas visitors, and with 24 million last year still had not returned to previous levels. The figures do not include visitors from Canada and Mexico, whose numbers are up substantially from 2000 but who tend to spend less than other international travelers to the U.S.

Part of the problem is the perception of frosty U.S. attitudes toward foreigners starting at customs, said Roger Dow, president of the Travel Industry Association. That and other factors make it difficult to attract more overseas travelers.

The U.S. should decode its complex entry rules and boost staffing at customs checkpoints, Dow and others said.

Vivian Dapal of EuroUSA, a travel agency that caters to groups from Europe, Asia and the Middle East, and Theresa Belpusi, who promotes Washington, D.C., with Destination DC, said American airports are particularly confusing.

"I don't think that people are questioning that we're trying to get our arms around security," Belpusi said. But she said the government should communicate better with "the people that are probably affected the most" — the tourists.

Frequent U.S. visitor George Somerville, of Glasgow, Scotland, said international flights are generally cheaper to places other than the United States.

"In the last 12 months, destinations my colleagues have traveled to include China twice, Singapore, India and Thailand," he said. "Much of that is to do with the price of flights — Air Asia, Emirates and Singapore airlines are doing great deals from the U.K."

Some in the travel industry also blame how the U.S. markets itself abroad.

Paula Bohaty, a group travel manager for the Nebraska Division of Travel & Tourism, said the industry would benefit from marketing smaller destinations that foreign travelers aren't familiar with and from pitching experiences foreign travelers find novel — like working on a farm.

Top destination cities are spending millions to promote themselves abroad and often compete with one another for foreign visitors, meaning less-obvious destinations with smaller marketing budgets have trouble being heard.

The industry is pushing a bill that would impose a fee on overseas travelers — to be matched by the industry, for about $200 million in all — to fund marketing to foreign visitors and communication of U.S. visa rules.