GENEVA - A World Health Organization committee has recommended approval for genetically altering the smallpox virus to make it easier to determine whether drugs to tackle the disease are effective, the U.N. agency said Thursday.

The alteration involves adding a marker gene to the virus that would glow green under a fluorescent light if the smallpox virus was still alive but would not react if it was dead, WHO spokesman Dick Thompson told The Associated Press.

"They recommended that experiments be done that would speed the screening of drugs for anti-smallpox activity," Thompson said.

Thompson confirmed this would constitute genetic manipulation - as reported earlier Thursday by National Public Radio - but stressed that the purpose of the experiments would be to try to improve smallpox treatment.

In the United States, however, a senior smallpox expert said he was wary.

"I think that it is unwise for us to be continuing research with a smallpox virus," NPR quoted Dr. Donald Henderson, President Bush's former bioterrorism czar, as saying.

Henderson ran the successful WHO campaign to wipe out smallpox in the developing countries of Africa, Asia and Latin America from 1966 to 1977.

The WHO committee, which met in Geneva Nov. 4-5, said further research should be carried out on the marker gene that would be inserted into the smallpox virus, Thompson said.

The World Health Assembly - the ruling body of the 192-nation WHO - would make a final decision on whether to approve the experiments.

"It will go through the bureaucratic process," Thompson said. "It will be a political decision."

The aim of the genetic alteration is to speed up the screening of anti-smallpox drugs and help to use up the last remaining stocks of the virus, which are being held in secure laboratories because the disease is so virulent.

The modified version of the virus would only be used in testing drugs for people who already have the virus and not for smallpox vaccines.

Today, the only smallpox vaccine available is unsafe for people with weakened immune systems, and can even seriously harm some healthy people, because it is made with a live virus called vaccinia that can spread through the body.

Smallpox is the only major disease to be successfully eradicated under a WHO-sponsored vaccination program. The last case was in 1978.

The disease is like a drastic case of flu, taking more than a week to show symptoms including fever and delerium. But, unlike flu, it leads to painful pus-filled rashes on the face and in the mouth, can lead to blindness, and can kill 30 percent of those who contract it.