Kenya Wildlife Service (KWS) transported critically endangered animals from Nairobi to Tsavo East National Park in the country's southeast last month, to boost the population of the beasts there, it said.
The World Wildlife Fund said in a statement: "Translocating wild animals of this size is extremely challenging and not without risk, but black rhinos are under enormous threat so efforts to try and better protect them, such as translocation, are crucial for future generations".
The Ministry of Tourism and Wildlife said salt poisoning may have caused the rhinos to perish as they struggled to adapt to saltier water in their new home.
According to Cathy Dean, chief executive of Save the Rhino, the relocation of endangered animals, a process called translocation, can help prevent their extinction.
It has suspended the ongoing move of other rhinos with the surviving ones being closely monitored.
Losing the rhinos is "a complete disaster", said prominent Kenyan conservationist Paula Kahumbu of WildlifeDirect.
"Rhinos have died. We have to say it openly when it happens, not a week later or a month later", she said.
FILE PHOTO: A female black Rhino stands in a box before being transported during rhino translocation exercise In the Nairobi National Park, Kenya, June 26, 2018. "We have to find out what's gone wrong, so that these mistakes are never repeated". The government agency has not said how the rhinos died.
The termed the incident as unfortunate affirming that disciplinary action will be taken, if the findings point towards negligence or unprofessional misconduct on the part of any KWS officers.
The seven were among 14 rhinos that were moved from Nairobi and Lake Nakuru national parks two weeks ago.
According to the Kenya Wildlife Service, nine rhinos were killed in Kenya a year ago.
Kenya had a rhino population of 1,258 in 2017 of which 745 are black rhinos, 510 are southern white rhinos and three were northern white rhinos, having grown from less than 400 rhinos in the 1980s.
Dean said there were once thousands of black rhinos in Kenya but now their population is in the hundreds. Three were shot dead inside a protected sanctuary in northern Kenya in May.
Black rhinos are considered critically endangered but its population has rebounded, although the species remains threatened due to poaching and habitat loss.