Google Inc. said it would expand its same-day shopping delivery service to three new U.S. cities and start charging customers for the service, which competes with Amazon.com Inc.

The Google Express service, which was earlier only available in certain parts of California and New York City, will be expanded to Boston, Chicago, and Washington D.C., Google said in a blog.

Membership for the service, which was earlier called Google Shopping Express, will cost $95 a year, or $10 a month.

Online retailer Amazon's same-day delivery service, called Prime, charges customers $99 per year, after a free one-year trial.

Google Executive Chairman Eric Schmidt, at a public speech made in Berlin on Monday, called Amazon its "biggest search competitor", the Financial Times and other media reported.

"Many people think our main competition is Bing or Yahoo. But, really, our biggest search competitor is Amazon," the FT quoted Schmidt as saying.

Schmidt said internet users are likely to go directly to the retailer if they are shopping.

Google's shares were up about 1.7 percent at $553.93 on the Nasdaq in early afternoon trading.