A 6.1-magnitude earthquake has struck northern Laos, shaking buildings as far away as the Thai capital Bangkok, around 800 kilometres to the south, US Geological Survey (USGS) says.

Shoppers have fled some of Bangkok's many malls in panic and some high-rise office blocks have been evacuated after they swayed.

The USGS says the quake hit shortly before 7:00pm almost 150 kilometres from the ancient Lao capital of Luang Prabang, a world heritage site on account of its centuries-old Buddhist pagodas and temples.

Luang Prabang residents say they felt only minor shaking.

Longitude and latitude coordinates given by the USGS places the quake's epicentre in the sparsely-populated, mountainous northern province of Luang Namtha, in the heart of the Golden Triangle opium-producing region near the Myanmar and Chinese borders.