SKYPE began showing callers on-screen ads today while they used the globally-popular free internet telephone service.

What Skype euphemistically referred to as "conversation ads" appear in calling windows of users who don't pay for subscriptions or have credits in accounts at the service.

"We're excited to introduce conversation ads as an opportunity for marketers to reach our hundreds of millions of connected users," Sandhya Venkatachalam said in a post at the official Skype blog.

"While on a one-on-one audio call, users will see content that could spark additional topics of conversation that are relevant to Skype users and highlight unique and local brand experiences."

The silent conversation ads are available to marketers where ever Skype is available and will be shown during free Skype-to-Skype calls on computer's powered by Microsoft's Windows software, according to Mr Venkatachalam.

Microsoft's Skype internet telephone service hopes to quadruple the number of users to get to one billion, division president Tony Bates said two weeks ago at the All Things Digital conference in California.

Mr Bates, who heads the unit that was acquired by Microsoft last year but operates autonomously, said growth will come from mobile users and from partnerships like the one Skype has with Facebook.

He cited Facebook as a key to growth for Skype, which now has 250 million users.

He said Skype could use the reach of Microsoft, the world's biggest software firm, to expand its presence, but without limiting itself to the Windows platform.

Skype users can make low-cost or free phone calls over the internet using their computers or smartphones. Skype bypasses the standard telephone network by channeling voice and video calls over the web.