The scene is familiar. The distraught vacationers are pleading pitifully for help. They've lost their travelers checks and they don't know what to do. The moral: always use American Express travelers cheques. "Don't leave home without them."

Well, one of American Express' competitors has some advice of its own for vacationers. "Don't leave home without the facts."

So says a full-page ad appearing in three major newspapers yesterday in which First National City Travelers Checks, a division of Citicorp of New York, claims that the American Express commercials depicting the confused couples are deceptive. The company notes that travelers checks other than American Express' -- notably First National City's -- can be refunded worldwide and says that the commercials, which have been running since February, should be stopped.

First National City said the commercials have damaged the credibility of other companies and caused it to lose sales. It is also considering a lawsuit for damages against American Express.

"The travelers check industry is in the peak season," said Ralph E. McEldowney, senior vice president for Citicorp Services, Inc. "By taking our message to the public we could bring pressure to bear and hasten the decision making process" to remove the American Express ads "on all levels."

"There's no major difference between our products except they have a bigger share of the market," McEldowney said.

But American Express, in a statement yesterday, checked its competitor tersely.

The giant would "back its ads to the hilt," according to Michael E. Lively, senior vice president of American Express Company and president of its travelers cheque division.

The commercials are based on facts and actual experiences, Lively said. In the last 60 days the American Express refund center in New York has received more than 300 calls from consumers who lost other brands of travelers checks, but called American Express because they didn't know what to do, the company statement said.

In a telephone interview Lively said that the compaign by First National City was maneuvered because "They're looking for some way to justify their drop in sales," he said. American Express travelers cheques have had an increase in sales over last year, Lively said, but he would not say by how much.

The Federal Trade Commission has received five complaints against American Express commercials since 1977, an FTC spokesman said yesterday. But Collot Guerard, deputy assistant director of the division of advertising practices, said from computer listings it is doubtful that the complaints concerned the current commercial.

The FTC however, will now "take a look at the American Express ads," Guerard said.

The agency said it "supports comparative advertising of this type. People are now going to go in and ask questions. That's better than the government getting in on this. It's a healthy debate going on."

The basis of First National City's complaint is that the commercials imply travelers will have trouble getting a refund for travelers checks other than American Express.

"We're not implying that no one else makes a refund," Lively said. "But we have the most extensive refund service by far and that's what we're trying to say."

The ads were cleared through the company's lawyers and its advertising agency lawyers, Lively said.

American Express travelers cheques are refunded at 60,000 outlets worldwide compared to 45,000 for First National City. Both have 24-hour emergency refund service and both claim to have the best system.

American Express, according to First National City, controls 55 percent of a $30 billion industry and shouldn't be such a bully in its ads, McEldowney said. First National City said it is ranked number two with 18 percent of the market share, but American Express says its competitor is number three.