The treaty, four years in the making, envisages total bans or restrictions on advertising and marketing, new labeling controls and a clamp down on smuggling and secondhand smoke.
Besides the treaty, the 192-nation health assembly, which opens Monday, will be dominated by the SARS virus and WHO's efforts to change global health regulations to cope with new infectious diseases and the threat of bioterrorism.
"Much to the surprise of many around the world, I am going to be supporting the tobacco treaty," Health and Human Services Secretary Tommy Thompson said on the eve of the meeting of health ministers.
But he stressed it would be a first step since the treaty still needs to be signed by President Bush and ratified by Congress.
Anti-smoking activists and developing countries have accused the United States of trying to undermine the tobacco accord.
The language of the treaty was agreed in March over U.S. objections that it did not allow countries to opt out of individual clauses — a procedure known as taking reservations.
The United States wrote to the other WHO member nations, saying that its ability to sign and ratify the convention was undermined by the ban on reservations and asking for support to reopen the negotiations and delete the ban. However, virtually no other country was willing to renegotiate the text.
"I'm not going to make any changes, no reservations," Thompson told a small group of journalists. "Our delegation here, headed by me, is in support of the tobacco treaty."
Sen. Tom Daschle of South Dakota and Rep. Nancy Pelosi of California, the Democratic leaders in the Senate and House respectively, asked Mr. Bush in a letter not to reopen the tobacco negotiations. Democratic Rep. Henry Waxman was also highly critical of U.S. behavior.
Until Sunday, most observers had feared that the United States would tell the assembly it wouldn't be able to support the treaty.
"Wow. That's astonishing," said Kathryn Mulvey, director of the campaign group Infact, when told of Thompson's declaration.
"That's a complete break from the past and it's great news for the rest of the world," she said.
The so-called Framework Convention on Tobacco Control is meant to stem the death toll in tobacco-related disease, which the health agency says stands at nearly 5 million people a year and is expected to climb to 10 million a year over the next two decades.
There are an estimated 1.2 billion smokers in the world and WHO surveys show that smoking rates among 13-15 year-old children are about 20 percent.
More than 400,000 Americans die each year from a smoking related disease — primarily lung cancer, heart disease and chronic lung disease — at a cost of more than $75 billion annually in direct medical expenses, Thompson said in February.
The U.N. health agency has won praise for tackling the spread of severe acute respiratory syndrome by issuing travel restrictions and advising affected countries on the course of action.
The WHO conference will feature sessions of everything from stopping violence and blindness, to sessions on controlling Pan African tsetse and trypanosomiasis, and a lecture titled: "Preventing 750,000 Measles Deaths."
The proposed tobacco treaty language was much tougher than expected as a result of a united front by developing countries who complain that they are falling victim to merciless campaigns by multinational tobacco companies.
It sets out steps for reducing smuggling and sales to minors, and directs signatories "in accordance with its constitution or constitutional principles, (to) undertake a comprehensive ban of all tobacco advertising, promotion and sponsorship."
Germany and China also joined the United States in March in expressing concerns about the treaty.
Some tobacco companies have targeted developing countries as smoking has become less popular in developed countries. In some cases, advertising has implicitly targeted minors.