The US Centers for Disease Control and Prevention (CDC) said it was aware of more than 700 global cases of monkeypox, including 21 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.
The main risk factor being close skin-to-skin contact with someone who has monkey pox sores
"There have also been some cases in the United States that we know are linked to known cases," Jennifer McQuiston, deputy director of the CDC's Division of High Consequence Pathogens and Pathology, told reporters on a call.
"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."
Monkeypox is a rare disease that is related to, but is 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.
Canada also released new figures, counting 77 confirmed cases - almost all of them detected in Quebec province, where vaccines have been delivered.
Though its new spread may be linked to particular gay festivals in Europe, monkeypox is not thought to be a sexually transmitted disease, with the main risk factor being close skin-to-skin contact with someone who has monkeypox sores.
A person is contagious until all the sores have scabbed and new skin is formed.
'More than enough vaccine'
Raj Panjabi, senior director for the White House's global health security and biodefense division, added that 1,200 vaccines and 100 treatment courses had been delivered to US states, where they were offered to close contacts of those infected.
There are currently two authorised vaccines: ACAM2000 and JYNNEOS, which were originally developed against smallpox.
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.
JYNNEOS is the more modern of the two vaccines, with fewer side effects.
"We continue to have more than enough vaccine available," Dawn O'Connell, assistant secretary for preparedness and response in the Department of Health and Human Services, told reporters.
In late May, the CDC said it had 100 million doses of ACAM200 and 1,000 doses of JYNNEOS available, but O'Connell said Friday the figures had shifted, though she could not divulge precise numbers for strategic reasons.
The CDC has also authorized two antivirals used to treat smallpox, TPOXX and Cidofovir, to be repurposed to treat monkeypox.
"Anyone can get monkeypox and we are carefully monitoring for monkeypox that may be spreading in any population, including those who are not identifying as men who have sex with men," said McQuiston.
That being said, the CDC is undertaking special outreach in the LGBT community, she added.
A suspected case "should be anyone with a new characteristic rash," or anyone who meets the criteria for high suspicion such as relevant travel, close contact, or being a man who has sex with men.