NEW YORK (AP) _ The Summer Olympics are 10 days away, but some corporate heavyweights already are tangled in marketing wrestling matches tied to the games.

Charge-card giant American Express announced Tuesday an advertising counteroffensive against Visa's Olympic ads, which use the same ''the Olympics don't take American Express'' line it used for the Winter Games.

Visa can make that assertion because it paid millions to become an official Olympic sponsor. The deal made Visa the sole payment card accepted at ticket offices and souvenir stands on the Olympics site.

But American Express said it has research that shows the Visa ads have left nearly one of four viewers with the incorrect impression that American Express cards are not generally accepted in Barcelona, where the games start July 25.

American Express said its new ads are aimed at correcting those misperceptions.

''To visit Spain, you don't need a visa,'' American Express says in one of the TV and print ads that debut Wednesday. A second commercial ends with the line, ''Obviously we're here for more than just the fun and games.''

''We simply cannot sit back and allow a competitor to create a lie about the value of our products and services in such a calculatedly vicious manner,'' Kenneth Chenault, president of the American Express Consumer Card Group, said in announcing the ad campaign.

Visa spokesman Brad Hennig offered no apologies for Visa's approach, which he said the San Francisco-based card provider has been using for five years.

''We stand by our ads. The ads are very clear. The Olympics don't accept American Express as payment,'' he said.

American Express was criticized by the International Olympic Committee last winter with similar ads that the card company said also were aimed at showing its cards were good at many sites in France, where the Winter Games were held.

The new campaign may arouse more debate about the value of Olympic sponsorship deals, and how sponsors use them in advertising.

United States Olympic Committee officials are examining a new Pepsi-Cola Co. commercial that features Olympian Magic Johnson, the basketball player who retired from the pros after discovering he had the virus that causes AIDS.

Pepsi, which is not an Olympics sponsor, has indicated its ad may be run during the games.

But Pepsi's rival, Coca-Cola Co., reportedly paid more than $30 million to be an Olympics sponsor. International Olympic Committee rules give U.S. authorities power to veto ads with Olympians from running during the games.

Another Olympic sponsor, athletic shoe maker Nike Inc., is embroiled in a legal dispute with a Spanish company that owns the right to the Nike name there.

Olympic committee officials are under intense pressure to protect official sponsors from marketing ambushes by non-sponsors.

Richard Pound, who oversees marketing activities for the IOC, said 12 companies including Visa and Coca-Cola paid the international group a total of $175 million for the right to be global Olympics sponsors.

The money, used for training athletes, preparing sites and conducting the games, would be more difficult to raise if others could create ad campaigns that leave viewers with the impression they may be sponsors.

''We gave given the sponsors exclusivity in their category. If you can't protect that ... what's the point?,'' Pound said.

Pepsi and American Express have denied their campaigns are marketing ambushes. Pepsi said it's had a long relationship with Johnson and the campaign was designed as a tribute to him. Amrican Express said it was only defending itself.

But Pound said American Express was trying to ambush Visa with last winter's advertising, and expressed disappointment over the new effort. He said he hadn't seen the American Express ads yet.

At the same time, Pound said Visa already has agreed to drop comparative advertising campaigns that name American Express outside the United States and has no plans to use it in the United States for the 1994 and 1996 games.

''It's not a campaign that I personally find the best in the world,'' Pound said. ''But it is one that Visa likes.''