There are more than 700 cases of monkeypox worldwide, according to estimates from the US Center for Disease Control and Prevention (CDC).
They include 21 instances in the United States, with investigations now suggesting it is spreading inside the country.
Sixteen of the first 17 cases were among people who identify as men who have sex with men, according to a new CDC report, and 14 were thought to be travel associated.
All patients are in recovery or have recovered, and no cases have been fatal.
"There have also been some cases in the United States that we know are linked to known cases," said Jennifer McQuiston, deputy director of the CDC's Division of High Consequence Pathogens and Pathology.
"We also have at least one case in the United States that does not have a travel link or know how they acquired their infection."
Six cases of monkeypox have now been confirmed in Ireland.
Monkeypox is a rare disease that is related to - but less severe than - smallpox, causing a rash that spreads, fever, chills, and aches, among other symptoms.
Generally confined to western and central Africa, cases have been reported in Europe since May and the number of countries affected has grown since.
Spanish health authorities reported 30 new cases of monkeypox today, bringing its total tally of infections to 186, the highest of the current outbreak.
Canada also released new figures today, counting 77 confirmed cases - almost all of them detected in Quebec province, where vaccines have been delivered.
France has detected 51 cases of monkeypox as of today, a rise from 33 confirmed cases on Wednesday.
Though its new spread may be linked to certain gay festivals in Europe, monkeypox is not thought to be a sexually transmitted disease.
The main risk factor being close skin-to-skin contact with someone who has monkey pox sores.
A person is contagious until all the sores have scabbed and new skin is formed.
Raj Panjabi, senior director for the White House's global health security and biodefense division, said 1,200 vaccines and 100 treatment courses had been delivered to US states, where they were offered to close contacts of those infected.
Though smallpox has been eliminated, the United States retains the vaccines in a strategic national reserve in case it is deployed as a biological weapon.
The European Union's drug watchdog is in talks with the maker of a vaccine against deadly smallpox to extend its use to monkeypox, a senior official said yesterday.
The European Medicines Agency said it was working with the manufacturer of the Imvanex jab "out of precaution" to ensure it had supplies if needed.
