W.H.O. members agree to begin talks on a global pandemic treaty.

The World Health Organization’s member states on Wednesday took the first step toward what many governments hope will be a legally binding treaty aimed at strengthening global defenses against pandemics.

A rare special session of the W.H.O.’s governing body agreed to set up an intergovernmental negotiating body that is to meet no later than March to begin negotiating an international agreement intended to ensure a more coherent and equitable response to future pandemics. But the United States and other countries have pushed for a weaker mechanism that would not carry legal obligations for member states.

The W.H.O. director-general, Dr. Tedros Adhanom Ghebreyesus, a strong advocate of a legally binding treaty, hailed the decision as historic, calling it “a once-in-a-generation opportunity to strengthen the global health architecture to protect and promote the well-being of all people.”

The decision marked only the beginning of what promises to be arduous negotiations to try to achieve consensus among the W.H.O.’s 194 member countries. The agreement calls for negotiators to deliver the result of their deliberations in May 2024.

The European Union and Britain have pushed for months for an ambitious treaty or convention that carries legal force. The discovery of the Omicron variant, which has prompted a new wave of travel rules and border closures, primarily targeted at southern African nations where the variant was first identified, has renewed criticism that countries worldwide are acting in a patchwork and discriminatory fashion.

“No better response to the emergence of the Omicron variant than this coming together of the international community behind the effort to strengthen the legal framework underpinning our collective response to pandemics,” Simon Manley, Britain’s ambassador in Geneva, said on Twitter.

The United States described the initiative in a statement as “a momentous step” but, with support from Brazil and other countries, it refused to commit to anything that is legally binding, and kept open the possibility of a weaker instrument.

The international agreement is intended to avoid any repetition of the “fragmented and splintered” steps by nations that Dr. Tedros has said weakened the global response to Covid-19. Proponents of a treaty want commitments to share data, virus samples and technology, and to ensure an equitable distribution of vaccines.

Those issues raise politically sensitive questions of national sovereignty over access to the sites of outbreaks, and potential investigations into origins of diseases — a source of tension between Western governments and China, which has resisted calls for an independent inquiry into the emergence of Covid-19 in the Chinese city of Wuhan in early 2020.

China said this week that it agreed “in principle with the ideas of further strengthening compliance, financing, sharing and information management.” But Beijing appeared wary of a new treaty and cautioned against “politicization, stigmatization and instrumentalization.”