Three workers have been killed and 18 injured after a tornado swept through a wood processing factory in Vietnam, local authorities reported Thursday.

On Wednesday evening, a tornado swept through a workshop owned by Thi Junma Ltd, a Chinese company located in Trung My Commune, Nguyen Van Toan, deputy chairman of the commune, told DPA via a phone call.

"The other 18 staff have sustained injuries and one is in critical condition now," he said.

The factory is based in northern Vinh Phuc province, some 60 kilometres from Vietnam's capital Hanoi.

The tornado triggered the collapse of the factory roof, he said, underneath which about 100 staff were busy working.

Thousands of pieces of wood and corrugated iron were flung high into the air, some reaching as high as 30 or 40 metres and spiralling dangerously before falling back to the ground.

Search and rescue forces found 21 workers beneath the fallen roof, three of whom were already dead.

Northern Vietnam is currently experiencing what could end up being its longest heatwave since 1993, the National Center for Hydro-Meteorological Forecasting told local media on Tuesday.

The heatwave is forecast to last until June 13 and may bring about more extreme weather as a consequence of the intense heat.

Hanoi and neighbouring provinces, including Vinh Phuc, also suffered from strong winds and heavy rains on Wednesday evening, causing flooding in some areas.