Apple Inc. on Friday announced that delivery of its white iPhone 4 will be further delayed until later this year due to manufacturing difficulties.

In a brief statement, the company said the white models of iPhone 4 "have continued to be more challenging to manufacture than we originally expected."

"As a result they will not be available until later this year," Apple said, without giving details.

"The availability of the more popular iPhone 4 black models is not affected," Apple added.

Sales have been brisk for Apple's latest smartphone, which first hit markets in the United States, Japan, France, Germany and Britain on June 26.

According to Apple, over three million units of iPhone 4 were sold until the middle of July.

After iPhone went on sale, consumers have been complaining of poor reception and abnormal levels of dropped calls when they hold the phone in a certain manner.

Faced with mounting pressure, Apple promised that it will offer free cases, or "bumpers," to all iPhone 4 buyers to help resolve the problem.

The program started on Friday and consumers have to download an application using their phone to receive the free case.

Consumers who bought the phones before July 23 must apply no later than Aug. 22, "otherwise, you must apply within 30 days of your iPhone 4 purchase," Apple said on its website.