The US will donate one million mpox vaccine doses and at least $US500 million ($A730 million) to African countries to support their response to the latest outbreak, a senior administration official says.

United States President Joe Biden will make the announcement in New York, where he is attending the United Nations General Assembly, and call on other countries to follow suit, the official told Reuters.

In August, the World Health Organisation declared mpox a global public health emergency for the second time in two years.

The declaration followed an outbreak of the viral infection in the Democratic Republic of Congo that has spread to neighbouring countries and beyond, including India, raising alarm.

"It wouldn't be a huge surprise if there were a case in the United States, either," the official said, noting that recent cuts in pandemic support by Congress could hamper a domestic response.

"We're really squeezing right now to be able to respond effectively in this moment and I think anticipating potentially additional cuts in the future will make it more and more difficult for us to have a robust response to these types of outbreaks."

Mpox can spread through close contact and while usually mild, it is fatal in rare cases.

It causes flu-like symptoms and pus-filled lesions on the body.

The outbreak in Congo began with the spread of an endemic strain, known as clade I but a new variant, clade Ib, appears to spread more easily through routine close contact, including sexual contact.

The expectation is that the US-donated doses will be of the Bavarian Nordic vaccine known as Jynneos in the US, and many will come from a stockpile in the US.

The one million doses are in addition to the 60,000 previously donated shots.

The US official said the administration expects Gavi, a public-private alliance that co-funds vaccine purchases for low-income countries, to distribute the donated vaccines.

"We feel really strongly that Gavi is the multilateral institution that is best positioned to procure and equitably distribute vaccines," the official said.

Last week, Gavi said it would buy 500,000 doses of Bavarian Nordic's vaccine, its first purchase to help battle the outbreak.

The Biden administration is also pushing for more vaccines to be manufactured in low and middle-income countries and is working with Brazil to identify how the Group of 20 club of nations can support the mpox response.

There is not currently a manufacturer in Africa able to make the complex drug substance needed to make the vaccine, the US official said.