Police revised an earlier death toll of 48 after several people died of injuries sustained during the attack.
"Four more people have died, in addition to the 48 who died on the spot," said regional deputy police chief Joseph Kitur.
Speaking earlier, Kitur said of the attack, which took place late Tuesday between the Pokomo and Orma peoples in the rural Tana River district: "It is a very bad incident.... They include 31 women, 11 children and six men."
"34 were hacked to death and 14 others were burnt to death," Kitur said, while several huts were torched after a gang of men launched the attack, the latest in a long history of bitter clashes between the rival groups in the remote area of Kenya.
It was not clear what sparked the attack, but the two communities have clashed before over the use of land and water resources, although the scale and intensity of the killings shocked police.
The attack happened in the Reketa area of Tarassa in Kenya's south-east, close to the coast and some 300 kilometres (185 miles) from the Kenyan capital Nairobi.
In 2001, at least 130 people were killed in a string of clashes in the same district and between the same two communities about access to land and a river.
"Clashes over pasture have been recurrent in this region," said national police spokesman Eric Kiraithe.
The Pokomo are a largely settled farming people, planting crops along the Tana River, while the Orma are largely cattle-herding pastoralists.
"Our investigations have shown that it is the Pokomo who attacked the Orma people, who live on an island" in the river, Kitur added.
Lawmaker Danson Mungatana, who represents the area, said the killings were "revenge attacks", adding there had been a string of tit-for-tat killings, attacks and cattle raids this month, though on a far smaller scale.
"There have been problems simmering for a while.... About 10 days ago three Pokomo were killed by the Orma community," he said.
"In revenge, the Orma raided villages occupied by the Pokomo and burnt down more than 100 houses. Now the Pokomo have once again revenged by killing about 50 people. These are purely revenge attacks."
Mungatana said that police had boosted numbers in the region since the attacks.
In 2007 Kenya spiralled into violence after contested elections that pitted candidates from different ethnic groups against each other, resulting in the deaths of some 1,200 people and displacing 600,000.