Imogen Foulkes BBC correspondent in Berne

The framework convention on tobacco control, which is being debated by more than 180 member states of the World Health Organisation (WHO), aims to impose a global ban on tobacco advertising and introduce new restrictions on the marketing of tobacco.

Poorer countries want a strict agreement

But opposition to an advertising ban, most notably from the United States, has put the agreement in jeopardy.

The US claims a total ban on tobacco advertising would be a violation of its constitutional commitment to free speech.

Health activists, however, accuse the US of trying to protect the tobacco companies, pointing out that the US is home to Philip Morris, the world's biggest tobacco exporter.

The negotiations in Geneva have been acrimonious.

Some anti-smoking groups have even urged the US to withdraw, saying there will be a better chance of agreement without Washington.

Funding fear

The framework convention is the first-ever attempt to draft a global anti-smoking treaty, and the WHO had high hopes for it.

The WHO estimates that smoking kills almost five million people every year, and it says 70% of future tobacco-related deaths will come from the developing world.

Poorer countries are pushing hard for a strict agreement.

But richer countries, like the US, object to the proposed ban on advertising and to a clause giving tobacco-control measures precedence over free trade laws in any future disputes.

Some WHO delegates claim the US has threatened to stop funding anti-smoking programmes if it does not get its own way on the convention.

US delegates deny this, but their opposition to an advertising ban remains firm.

The WHO says the negotiations must end on Friday, but agreement still seems very far away.