BANGKOK (AFP) - Ten students were missing after a ferry carrying primary school children sank on a tributary of the Mekong River in Laos on Tuesday, state media said.

Officials rescued 20 students after the ferry sank at 7.30am on the Nam Ngum river in the Sinxay region, but 10 students were still unaccounted for, according to the official Lao news agency KPL.

"Officials are still investigating exactly what happened," said Mr Kansam Teangmaniwoung, an official in the Ministry of Information, Culture and Tourism, as cited by KPL.

It was not immediately possible to confirm any further details.