Charles Schwab Corp's automated investment product has attracted $1.5 billion of assets in more than 23,000 accounts in its first six weeks, about 20 percent of who are new clients, Chief Executive Officer Walt Bettinger told analysts on Thursday.

The pace of sign-ups for the Schwab Intelligent Portfolios, a "robo-advisor" that allocates cash among exchange-traded funds according to formulas based on client questionnaires, has slowed after an initial burst, but the percentage of clients that are new to Schwab is growing, Bettinger said in a business update.

The volume of traditional client stock trading remains low, he cautioned, while profit from investing client cash is weak because of low interest rates that Schwab had expected would be higher by now. The San Francisco-based company last week reported a 7 percent decline in its first-quarter net income.

TD Ameritrade Holding Corp., which competes with Schwab for discount brokerage customers as well as for independent investment advisers with wealthy clients, also blamed persistently low interest rates earlier this week for lower-than-expected quarterly results.

Both companies said they will continue to invest in new technology and products, lifting their expenses in the short term even as revenue is constrained by low rates. Schwab pulled back on some projects in 2014 when it became clear that the Federal Reserve was not raising interest rates, but is more committed to its spending plans this year, executives said on the conference call.

When rates do rise, Schwab and TD Ameritrade said their earnings should skyrocket because the money they earn from investing customers' cash will rise faster than what they pay for that cash. Asked if the new "robo" product was cannibalizing such accounts, Schwab executives said about 11 to 12 percent of the $1.5 billion in Intelligent Portfolios came from advised accounts at the company.

Bettinger also said that Schwab has been building its overall market share by attracting clients from large wirehouse firms such as Morgan Stanley Wealth Management and Bank of America's Merrill Lynch Wealth Management.

Schwab has modified its business model in recent years from pure discount brokerage to a full-service model that offers advice to customers in return for fees.

As of March 31, it was the largest brokerage firm as measured by its more than $2.52 trillion in client assets, Bettinger said.