Starbucks Corp. on Tuesday said it is aiming to make the milk and other dairy products it serves in its U.S. coffee houses free of a controversial artificial growth hormone used in dairies to increase milk production.

A conversion would initially be aimed at all 5,500 U.S. company-owned stores, but Seattle-based Starbucks is also exploring such a move with more than 3,000 licensed locations, a company spokesman said.

The move comes after Starbucks was targeted in a campaign by consumer groups critical of the use of an artificial hormone known as rBGH, which is given as a supplement to dairy cows to increase milk production.

"We are actively engaged with all our dairy suppliers to explore converting our core dairy products to be rBGH-free in our U.S. company-owned stores," Starbucks spokesman Brandon Borrman told Reuters. "It is something we're aiming for."

Borrman said the company this month had already boosted the percentage of its supply that is free of the hormone to 37 percent from 27 percent at the end of 2006.

"This is something that our customers have requested," Borrman said. "It is just making sure our suppliers can supply the amount of milk we need."

Borrman said so far the company has been able to provide the artificial hormone-free dairy products in Northern California, New England, New Mexico, Montana, Oregon, Washington, Idaho and Alaska.

The dairy products involved include fluid milk, half-and-half, whipping cream and eggnog.

Borrman said there is no similar move for overseas locations because the supplement is not allowed in "most other nations."

Critics say that rBGH given to dairy cows increases the level of another growth hormone in both cows and humans known as IGF-1, and that elevated levels of IGF-1 has been associated with increased cancer rates. Critics also say growth hormone supplements are harmful to the dairy cows.

Proponents of the hormone supplement say there are no health concerns for humans or the cows, and milk products from cows given the supplement are no different from milk from cows that don't receive the supplement.

Patty Lovera, assistant director for Food and Water Watch, who pushed Starbucks to make a change, said it was "encouraging" to hear of the company's efforts.

"We want to see them actually get it done," Lovera said.

Earlier this month, Starbucks said it was making sure all of its pastries and other foods sold at half of its U.S. outlets would be free of artery-clogging trans fats.