GENEVA -- The head of the World Health Organization on Monday voiced confidence that countries would one day reach a deal on a pandemic accord after failing to produce an agreement last week, although health officials warned it could take years.

Health officials have voiced frustration with the negotiations that have been dogged by lengthy discussions past midnight, last-minute shifts in position and growing criticism from right-wing commentators that the treaty would undermine sovereignty, which the WHO strongly denies.

In a symbol of the resistance, a truck with a sign reading "NO to the Pandemic Treaty. STOP the UN Power Grab" was seen near the U.N. headquarters in Geneva where the talks are taking place.

Ministers from among the WHO's 194 member states are seeking to wrap up more than two years of negotiations on new rules for responding to pandemics during the May 27-June 1 World Health Assembly after COVID-19 killed millions.

Negotiators failed to produce a draft deal on Friday for formal approval by the assembly this week.

"Of course, we all wish that we had been able to reach a consensus on the agreement in time for this health assembly, and cross the finish line," Director-General Tedros Adhanom Ghebreyesus said in his opening address.

"I remain confident that you still will, because where there is a will, there is a way. I know that there remains among you a common will to get this done."

A senior U.S. administration official said Washington remained committed to the process but estimated another 1-2 years of talks. "There's a lot of frustration," she said. A health diplomat said reforms to the process were being considered as well as an extension of between 5-24 months.

Negotiations are still ongoing for an update to existing health rules on outbreaks and negotiators say that a deal is close, including on a new tiered system of alerts following criticism the WHO was too slow to declare a COVID emergency.

Lawrence Gostin, a professor at Georgetown Law in Washington D.C. involved in the negotiations, said the outcome was a "shameful abdication of political leadership" but voiced confidence the reforms would pass this week.

"They have enormous potential value for making the world safer and more equitable," he told Reuters.

(Reporting by Gabrielle Tetrault-Farber and Emma Farge; Editing by Sriraj Kalluvila and Nick Macfie)