Three mysterious metal spheres that fell from the sky and landed in northern Vietnam on Saturday have been seized by the Vietnamese military and are being investigated, Vietnamese media report.

Witnesses reported hearing thunder-like noises in the sky and seeing the objects flying up there early Saturday morning before they crash-landed, Thanh Nien News reported.

Two of the metal balls, one 250 grams and one six kilograms, landed in Yen Bai province. The smaller one hit the roof of the house, while the larger, about the size of a basketball, landed in a garden.

The third and largest by far — around 35 to 45 kilograms and about the size of an exercise or stability ball — fortunately landed away from people, near a stream in a maize field in neighbouring Tuyen Quang province.

All three were seized by Vietnam's Air Defence Arms, and are being investigated by the Ministry of Defence, VietnamNet Bridge reported Wednesday. The ministry's findings will be reported to the International Civil Aviation Organization.

The largest ball landed in a maize field near a stream, away from people. It appears to be a compressed air tank that may be from a rocket launch. (Thanh Nien News)

So far, investigators have determined that the objects are specialized compressed air tanks that may be from a rocket and that they were manufactured in Russia, although they may have subsequently been sold to other countries. They're made of metal and don't contain radioactive or explosive materials.

Earlier, Nguyen Khoa son, a professor from Vietnam's State Space Science and Technology Program, told VietnamNet Bridge that the objects might be the result of a failed satellite launch. He noted that they were intact, suggesting they fell from an altitude of less than 100 kilometres.

Strange spheres like this have previously fallen in other places around the world. Most recently, some were reported in Spain and Turkey in November.