The children were playing football when they found the shell near a rice field in the village of Khanh Phuoc, in the south-central province of Binh Dinh, on Monday.

Five of the children, aged between 10 and 13, were killed instantly and a sixth died in hospital on Tuesday, police said.

The United States dropped hundreds of thousands of cluster bombs on Vietnam during the country's war, which ended in 1975, many of which did not explode.

The two injured children are in a critical condition in hospital, said Luong Van Ngan, police chief of Phu Cat district.

He said the mortar shell had probably been dredged up from a canal during road works and exposed by rain.

War legacy

Thousands of Vietnamese people been killed or maimed by bombs, land mines and artillery shells left over from the war.