The A40 has been closed in both directions due to a fallen tree and snow.
The incident is mainly affecting traffic between the B4344 Stone Street, in Llandovery, and Halfway.
It is believed that the road will most likely be closed for several hours.
Police have advised to avoid the area.
A Welsh Ambulance Service spokesman confirmed that crews had not attended the incident but had been very busy throughout the day.
Rhian Gowen witnessed the tree falling on the main road.
She said: “This tree came down in front of us and crushed the camper van.
“We were just shocked as it fell on the van right in front of us and we had only just moved from the very spot it fell.
“Luckily all are fine.
“Only a police dog handler turned up after over half an hour.
“Like a half way house here! Snowing heavy now.”
Elsewhere, flooding has affected parts of the country, including Port Talbot where a stream reportedly burst its banks.
Near Bridgend, in Kenfig Hill, two women had to be rescued by fire crews after their car became stuck in water.
