Three boys, ranging in age from 6 to 11, and a 4-year-old girl were all killed in southern Vinh Long City Sunday, local police said. The shell was hidden in a bamboo stand off a small road near a local man's home. Nguyen Van Lat, 49, was hosting a party for relatives on the anniversary of a relative's death. The children had gone out to play when party-goers heard the loud explosion, the website Tuoitrenews.vn reported.
Two other men were injured in the blast, but were expected to live.
Police said Lat knew of the mortar shell and did not report it to police.
Since the end of the conflict in 1975, more than 40,000 Vietnamese have been killed by leftover unexploded bombs from the conflict and another 60,000 have been injured, the news website said.