The COVAX vaccine-sharing facility urgently needs 190 million doses by the end of June to fill a gap left by India halting vaccine exports, as coronavirus cases surge in South Asia, the GAVI vaccine alliance and World Health Organisation say.
"We need an additional $US2 billion ($A2.6 billion) to lift coverage in (advance market countries) up to nearly 30 per cent, and we need it by June 2 to lock in supplies now so that doses can be delivered through 2021 and into early 2022," they said in a statement.
"At least one billion doses could be shared by wealthy countries in 2021," it said.
The appeal comes as UNICEF said it had signed a conditional supply agreement to procure up to 220 million doses of the Russian-made Sputnik V vaccine by later this year.
The deal is contingent on the vaccine being approved for emergency use by the WHO and on GAVI signing a purchase agreement to buy vaccines on behalf of the UN-backed initiative known as COVAX.
Sputnik V is currently being assessed by the UN health agency for safety and efficacy.
Research published in the journal Lancet this year suggested the vaccine is about 91 per cent effective.
The shot is currently used in numerous countries.
In a statement on Thursday, UNICEF says it is "ready to deliver as soon as regulatory milestones have been met".
The announcement could help bolster stocks for the COVAX effort, which aims to distribute COVID-19 vaccines to developing countries.
The vast majority of COVAX's supplies are from the Serum Institute of India, which is keeping most of its vaccines to deal with a coronavirus surge of cases and deaths.