The United States has declared a monkeypox outbreak a public health emergency, Health and Human Services Secretary Xavier Becerra says, in a move expected to free up additional funding and tools to fight the disease.

The World Health Organisation has also declared monkeypox a "public health emergency of international concern," its highest alert level.

The WHO declaration last month was designed to trigger a coordinated international response and could unlock funding to collaborate on sharing vaccines and treatments.

US President Joe Biden on August 2 appointed two top federal officials to coordinate his administration's response to monkeypox following declarations of emergencies by California, Illinois and New York.

Cases have crossed 6600 in the United States, as of Wednesday's data, almost all of them among men who have sex with men.

First identified in monkeys in 1958, the disease has mild symptoms including fever, aches and pus-filled skin lesions, and people tend to recover from it within two to four weeks, according to the WHO.