American Express Co. (AXP) said on Wednesday that it was worried some Wall Street analysts were being overly optimistic about its short-term earnings outlook.

Speaking at an investor conference hosted by Merrill Lynch, Ken Chenault, the chair and chief executive of the U.S. credit-card and travel services giant, said that some analysts appeared to believe the company would report earnings-per-share growth from continuing operations in excess of 25 percent.

Chenault called those estimates "far too high" pointing out that they exceeded the company's long-term growth targets of 12 percent to 15 percent.

"I do appreciate the optimism that's been out there," Chenault said, "but I can tell you that number is far too aggressive."

Chenault's comments sent American Express shares down $1.43, or 2.81 percent, to $49.50.

Chenault said he rarely commented on consensus earnings estimates. But he said the company's spin off earlier this year of Ameriprise Financial Inc. (AMP) may have confused analysts and so he felt compelled to break the rule.

"The divergence was such that, given some of the complications of the spin-off, we think people need to look back and make sure they have the right basis," he said.

During the conference, Chenault also detailed the effect that the recent spike in bankruptcy filings prompted by a change in U.S. law would have on American Express' future results.

He said the company currently expects fourth-quarter bankruptcy-related write-offs in its U.S. consumer and small-business portfolios to exceed last year's write-offs by somewhere between $200 million and $275 million.

Chenault also told the conference American Express would continue to spend aggressively on marketing, saying the outlays were proving to be very effective in helping the company penetrate target markets.

"If I'm getting results, why would I cut back on marketing when we're in an environment where we're growing share faster than we ever have?" he said. "Why would I do that?"