The bus, which was also carrying some timber, burst into flames in the early morning, a guard from central Nghe An province told AFP on the condition of anonymity, without elaborating.
It is common for passenger buses in the remote region to be packed with goods.
According to VNExpress news site, the 35-seat bus was 10 kilometres from the border with Vietnam when the incident happened.
"After a huge bomb-like explosion, the bus was torn into pieces," the site quoted passenger Pham Dinh Tu as saying.
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The report added that Laos authorities confirmed the dead were Vietnamese nationals working in Laos.
Several other injured victims are reportedly receiving care at a hospital in Vietnam.
Officials are still investigating the cause of the accident, the report added.
Laos state media carried no reports about the incident on Thursday.
The communist nation heavily censors all media, restricts foreign journalists and rarely publishes negative reports.