By Christina Farr

SAN FRANCISCO (Reuters) - Apple Inc on Tuesday released a report on employee diversity, and its numbers are similar to those of other Silicon Valley companies, prompting Chief Executive Tim Cook to say there is still work to be done.

The employee survey comes on the heels of recent reports from technology companies Google Inc and Twitter Inc, but it is unique in one significant way. It alone is accompanied by letter from a company CEO, in which Cook stresses the company's commitment to being "innovative in advancing diversity."

Blacks and Hispanics make up about 18 percent of Apple's workforce, a ratio that is about triple of those of most other tech firms. Nine percent of its workers did not disclose their ethnicity.

Apple breaks down the numbers into three categories: leadership, technology and non-technology. The technology category, which is 80 percent male, includes Genius Bar employees and engineers.

The numbers include its large contingent of store management employees. Apple runs 254 retail stores in the United States and 427 globally, according to its most recent quarterly report.

"As CEO, I’m not satisfied with the numbers on this page," he wrote. "They’re not new to us, and we’ve been working hard for quite some time to improve them."

But Cook noted that Apple's definition of diversity goes beyond traditional categories such as race and gender. "It includes personal qualities that usually go unmeasured, such as sexual orientation, veteran status and disabilities," he wrote.

The diversity reports have spurred a national debate about the lack of diversity at Silicon Valley's tech companies and how to improve the ratio. At Google, some 70 percent of employees are also male, and 61 percent are white. Twitter's overall employee population is 70 percent male and 59 percent white.

While Apple's numbers are similar to those of its competitors, some experts say that the company is a step ahead of the rest.

"Apple will do everything it can to make their workforce look more like the population they serve," said Fred Sainz, vice president of communications and marketing at the Human Rights Campaign. For 13 years running, the HRC has awarded Apple a perfect score on its corporate equality index, which rates American workplaces on LGBT equality.

After taking the reigns, Cook promoted Cuban-American Eddy Cue to a leadership role at Apple, and brought on former Burberry chief executive Angela Ahrendts. The company also recruited Lisa Jackson, the first African American to head up the Environmental Protection Agency, to run its environmental efforts.

In recent years, Apple executives have spoken out publicly in support of a variety of social and environmental causes, including diversity, accessibility and human rights. Cook made an appearance at the San Francisco Pride Festival for the first time this summer to cheer on thousands of employees and their families who showed up. (http://www.reuters.com/article/2014/06/29/usa-gay-tech-idUSL2N0PA0IW20140629)

(Reporting By Christina Farr; Editing by Steve Orlofsky)