WASHINGTON (Reuters) - U.S. troops have found a laboratory under construction in southern Afghanistan where al Qaeda guerrillas apparently planned to produce biological weapons, the United States said on Saturday.

U.S. officials told Reuters the facility near the bustling city of Kandahar was apparently being built to produce deadly anthrax and other agents when it was found earlier this month.

"It wasn't completed, so they weren't at the point of producing weapons yet. There were no samples of biological agents," Air Force Maj. Bill Harrison, a spokesman for the military's Central Command, said in response to questions.

He said in a telephone interview from command headquarters in Tampa, Florida, that the laboratory was found by troops operating near Kandahar, but the discovery was not connected to a major military operation in Afghanistan's eastern mountains.

The United States has warned repeatedly that fugitive al Qaeda leader Osama bin Laden wanted to obtain weapons of mass destruction. Defense officials said on Saturday the discovery of the unfinished facility was the strongest sign yet of such planning.

"It contained medical supplies and commonly available laboratory equipment suitable for growing biological samples as well as a variety of associated supplies," Harrison told Reuters.

He and other officials declined to provide additional details, including the exact site of the laboratory.

The White House said on Saturday that there was no evidence that the militant Islamic group headed by bin Laden, accused by Washington of masterminding Sept. 11 hijacked airliner attacks on America, had obtained anthrax.

NO EVIDENCE OF ANTHRAX YET

"We have no evidence that they've acquired anthrax, but we've always said that we know that al Qaeda wants to acquire biological and nuclear weapons. So none of this would surprise us," White House spokeswoman Claire Buchan told Reuters.

"This is why we need to be diligent in waging this war" on terrorism, she said.

A British government source told Reuters in London on Friday that U.S. troops had found a biological laboratory in the mountains of eastern Afghanistan near Gardez during a major military thrust this month against regrouping al Qaeda and Taliban fighters.

But Harrison said such a facility was not found in that area, suggesting the source had simply given the wrong location. U.S.-led forces recently wound up a major assault, called "Operation Anaconda," in the region.

The British source, who spoke on condition of anonymity, said the evidence of biological weapons was a major factor behind Britain's sudden decision this week to send up to 1,700 elite mountain troops to reinforce the U.S.-led coalition waging the war on terror.

Although Defense Secretary Donald Rumsfeld has said there is ample evidence that al Qaeda wanted weapons of mass destruction, U.S. defense officials said earlier this week that no evidence of actual biological, chemical or nuclear arms had been found to date in about 60 al Qaeda and Taliban sites searched by U.S. troops in Afghanistan.