About 100 people are isolating after 14 coronavirus cases were linked to a sports club awards evening in Carmarthenshire.
Police and councils have warned Carmarthenshire, Pembrokeshire and Ceredigion could be put into lockdown if people did not social distance.
It comes after about 60 to 80 people went to an event at Drefach Cricket and Football Club.
Carmarthenshire leader Emlyn Dole said people were "putting lives at risk".
Mr Dole told BBC Wales that a few people's actions were risking other people's health, making him "very angry".
The council said the event at the sports club was held on 29 August and about 100 people were having to self-isolate for 14 days.
In a statement, issued on Monday, the Drefach FC said it had suspended training until further notice after some senior players tested positive for Covid-19.
"We immediately advised Public Health Wales and are liaising with them on a continuing basis," it said.
"We are awaiting further advice from them and assure you that we will update you further as soon as it is received."
In a joint statement with Dyfed-Powys Police, Mr Dole, alongside the leaders of Pembrokeshire and Ceredigion councils, said: "The last thing we want to do is go into a local lockdown, as has already happened elsewhere in Wales.
"But, if the rising threat and lack of social distancing continues we may have to take necessary action.
"This pandemic is far from over - the virus is still circulating and the risk is still high. People need to respect the measures that are in place and take personal responsibility for social distancing and good hygiene."
Dr Giri Shankar: "We manage any clusters of Coronavirus appropriately, including by providing advice around infection prevention and control, and by supporting contact tracing where required."
Chair of Hywel Dda University Health Board, Maria Battle, called on people to maintain social-distancing, saying the virus had "not gone away".
"We have been very fortunate in west Wales in our relatively low infection rates of Covid-19," she said.
"This was due to the tremendous efforts of the public in following safety guidelines which prevented the spread of this deadly disease."