With the United States gripped by an anthrax scare possibly linked to Osama bin Laden's al-Qaeda terrorist network, investigators have launched an international hunt for sources of the disease. Cases have been detected in Florida, Nevada, New York and Washington; one US journalist has died as a result; and several other people have been infected, including a seven-month-old boy.

The FBI says it has not yet determined who is attempting to spread the disease by sealing powder in envelopes. Links have been drawn to bin Laden by threats he has issued, and some of the envelopes containing the bacteria were mailed from places where those responsible for the September 11 attacks on the US lived or visited.

Once this threat to America ends, experts believe one benefit of the scare will be that it will boost previously half-hearted international efforts to eliminate the threat posed by these potential weapons of mass destruction.

North Korea, the rogue isolationist state which has had previous dealings with terrorists, is already being reported as a suspect in supplying anthrax to the al-Qaeda network. It has a rudimentary infrastructure capable of producing infectious biological agents such as anthrax, bubonic plague and cholera, according to a US Defence Department report released in January.

Pyongyang signed an international treaty designed to prohibit the development and production of biological weapons but had pursued the capability to produce them since the 1960s, said the report.

Also suspected as possible sources are Iraq - which, following the 1991 Persian Gulf War, was found by the United Nations to have been experimenting with anthrax - and former Soviet laboratories in Russia or Central Asia.

China has been accused by some in the US of having a biological-weapons programme sophisticated enough to have the ability to produce anthrax. However, the exact nature of Beijing's alleged programme remains unclear, since evidence on such a sensitive subject is inevitably sketchy and shrouded in secrecy. Beijing vehemently denies producing biological weapons and claims it has only developed anti-germ-warfare technology in order to defend itself in the event of an attack.

A defence white paper released by China last year says it has 'fully and conscientiously fulfilled its obligations' under the 1972 Biological and Toxic Weapons Convention.

'Having suffered grievously from biological-weapon attacks in the past, China supports work that helps comprehensively to strengthen the effectiveness of the convention,' the paper said, referring to Japan's horrific germ-warfare research on the mainland in the 1930s, when the infamous 731 military unit unleashed bubonic plague, cholera, dysentery and typhus in attacks on civilians and soldiers.

But the January US Defence Department report claims Beijing not only has the technology to make such weapons but also maintains at least some parts of a programme enabling it to do so. 'China continues to maintain some elements of an offensive biological-warfare programme it is believed to have started in the 1950s,' said the report, which outlines the Pentagon's assessments of proliferation threats for weapons of mass destruction around the world.

It said the PLA's biological-warfare capability was believed to have been based on technology developed before Beijing signed the convention in 1984 and that the PLA had the expertise to mount biological agents on weapons such as missiles.

'Since 1984, China consistently has claimed that it never researched, produced or possessed any biological weapons and would never do so,' said the report. But it alleged that information supplied by Beijing to other nations for verification under the convention might be incomplete or inaccurate.

Another report from a Washington think tank also claims US intelligence agencies have uncovered evidence that China is pursuing biological research at two centres run by civilians and controlled by the PLA. The research paper, compiled by the Chemical and Biological Weapons Non-proliferation Project of the Henry L. Stimson Centre, did not identify the two centres, which were 'known to have been previously involved in the production and storage of biological weapons'.

But a Taiwanese expert identified Lanzhou, capital of Gansu province, and Beijing as the locations. Andrew Yang Nien-dzu, secretary-general of the Taiwan-based Chinese Council of Advanced Policy Studies, said the research at both units could be used not only to protect troops and civilians from an enemy but also to develop biological weapons.

An Anti-Biological-Warfare Unit reportedly operates from the Military Medical Research Institute of the Beijing military region, according to a 1994 report in the Chinese-language newspaper Ming Pao. It said the institute looked like a villa from the outside but inside were tens of thousands of deadly insects in air-conditioned rooms. Soldiers wore white protective clothing covering all of their bodies. Signs warning 'Deadly Bacteria Laboratory' hung on glass doors. A room with tens of thousands of flies and mosquitoes living on milk bran and animal blood was classified as the most dangerous in the building. Shelves were full of wooden boxes containing deadly insects.

The then-head of the unit, Fu Genming, was reported as saying: 'The PLA does not have an offensive 'biological-warfare unit' or 'bacteriological-warfare unit'. But it does have an anti-biological-warfare unit. All of our research is open to the whole world. It is an angel of world peace and health for mankind.'

But researcher Eric Croddy, of a Californian think tank, the Monterey Institute of International Studies, believes this institution might also have been responsible for isolating a deadly disease that is reported to have broken out in Xinjiang province in the late 1980s and which a former Soviet biological-weapons-programme chief claims was caused by mainland research into biological warfare.

Ken Alibek, a scientist who defected to the US from Russia in 1992, says Soviet intelligence found evidence of two epidemics of hemorrhagic fever near Lop Nor, an area where the disease was previously unknown. 'Our analysts concluded they were caused by an accident in a lab where Chinese scientists were weaponising viral diseases,' Dr Alibek wrote in his book on the Soviet biological-weapons programme, Biohazard.

Beijing had declared under the biological-weapons convention that it had a 'national defensive biological-warfare research and development programme', said Mr Croddy.

Its declaration of facilities stated that the Institute of Microbiology and Epidemiology at the Academy of Military Science in Beijing had a dual-use capability and was used for biological-warfare-defence research, he said. Beijing also listed seven vaccine-production facilities around the nation.

Taiwan is also suspected of having biological weapons, according to the Stimson Centre. 'Taiwan is said not to have biological weapons, but it continues to manifest an active interest in conducting biological research of a military-applied nature,' said its report. It said there was not enough evidence to determine whether Taiwan was producing biological agents or putting them into weapons, but its scientific and research base was advanced enough for the production of germ-warfare arms.

Mr Yang, of the Taiwan-based Chinese Council of Advanced Policy Studies, said Taiwan's biological-warfare research was carried out at a chemical-warfare-research unit at Chungshan Institute of Science and Technology, a military weapons-development facility.

The implementation of the biological-weapons convention has been hindered by an inability to reach agreement among nations about suitable ways to inspect facilities and other verification measures, according to experts.

The international effort to tackle germ warfare had been less elaborate and had not attracted as much energy as the fight against nuclear weapons, said Gary Milhollin, director of the Wisconsin Project on Nuclear Arms Control. But biological-weapons proliferation was likely to attract more attention in the wake of the US incidents, he said.

Stimson Centre researcher Leslie-Anne Levy said it would take time to see whether the attacks would affect Washington's position on a monitoring protocol after it recently rejected proposals for inspections of facilities and other issues on the grounds that they could jeopardise national security and confidential business information. 'At this point, what people are worried about is terrorism. When you are talking about a terrorist group, they are not going to sign on to a treaty.'

Mr Milhollin said the best way to eliminate the threat was to get rid of germ agents and nations' capabilities to wage war with them.

China has criticised America's scrapping of monitoring proposals for the convention. 'The position of one country, which possesses nearly half of all the bio-industry and bio-defence facilities in the world, is making substantive negotiations impossible,' said Chinese ambassador for disarmament Hu Xiaodi in August.