By Richard Black BBC science correspondent in Paris

Protesters want money spent now

US Health Secretary Tommy Thompson says the funding deficit could be as much as $800m.

On Wednesday, the final day of the International Aids conference in Paris will discuss possible remedies.

Mr Thompson will share a table with health ministers from Africa, the heads of the WHO and UNAids and campaigners from groups like the South African Treatment Action Campaign.

Funding drying up

The fund was established three years ago as the definitive global response to the three major developing-world diseases.

Heads of the world's most powerful governments pledged to use the fund to finance effective treatment and prevention.

Initially, dollars and euros came in by the million, but for more than a year there have been rumours that the fund was running out of cash.

Activists here have been saying the shortfall is around $600m, but Mr Thompson says the exact figure isn't yet clear.

"It's a ballpark figure - nobody knows for sure," he told delegates in Paris.

Estimated Aids spending for 2003: Total: $4.7bn $1.6bn - Bilateral aid $1bn - Multilateral aid and Global Fund $1bn - Governments of affected countries $1.1bn - Household spending in affected countries

"We won't know the exact amount until we find out how many more contributions come in, but we will be short."

The fund has around $300m in cash, but over the next few months it will approve expenditure on a substantial number of projects.

The exact shortfall depends on how many projects it approves, but Mr Thompson has said it could reach $800m.

Appeal

Donors haven't pledged as much as the fund needs, and not all of the money which has been pledged has made it into the fund's coffers.

Mr Thompson appealed to governments, companies and charitable foundations to contribute more money urgently.

Thompson: Appeal for funding

On Wednesday, the fund holds a supporters' meeting here to discuss the problem.

US President George W Bush has said he will provide $1bn if Europe matches that figure.

The European Commission President, Romano Prodi, will be coming to the meeting, but observers say a big European commitment is unlikely.

Activists say the developed world needs to prove its commitment to Aids by finding the money quickly.

Failure to come up with the money, they say, will prove the West does not care about diseases of poverty.

Scientists here are also pressing for action, circulating a petition calling for money to make the drugs they have designed available to all.