More than 700 global cases of monkeypox, including 21 in the US have been detected so far, said the US Centres for Disease Control and Prevention (CDC).
CDC said investigations suggest that the disease 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 US 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,” she said.
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.
Canada also released new figures on Friday, 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.