(CNN) -- Most of 26 refugees on board an overloaded boat which capsized in a Sudan river Wednesday are believed to have drowned, the United Nations refugee agency said.
Three children and seven women are among the 21 unaccounted for and believed dead, UNHCR added.
They were on a packed boat that tried to cross the Atbara River -- a eastern Sudan tributary of the Nile River -- at 1:30 a.m. local time along with three other vessels, U.N. High Commissioner for Refugees said in a statement.
"(This) tragic incident highlights the plight of refugees in eastern Sudan and the inherent risks of smuggling people," the statement said.
The boat was carrying 26 Eritrean and Somali passengers -- including eight women and at least three children -- although the vessel was only meant for 15 people, according to UNHCR.
Four Eritrean men swam to safety along with a Somali woman who clung to a floating log, after the vessel capsized about a third of a mile (700 meters) from shore.
None of the 21 missing have been accounted for, although UNHCR said "arrangements have been made for a dignified burial of the bodies, if and when they are recovered."
One of the male survivors said he was trying to reach the Sudanese capital, Khartoum, in search of work.
He said he arrived at the Shagarab refugee camp in eastern Sudan from Eritrea about three weeks ago. He said he and several others paid about $100 each to travel across the river and then reach Khartoum by road.
Two suspected smugglers, who are also refugees, are in police custody, UNHCR said.
Approximately 130,000 refugees from Eritrea, Ethiopia and Somalia live in eastern Sudan, UNHCR said.
Most of them receive international assistance at refugee camps, but 40,000 "eke out a living in rural and urban concentrations," it said.
Some have been in the "protracted situation" for as long as 40 years, and "a steady flow of refugees" from Ethiopia, Eritrea and Somalia continues to flood Sudan, according to UNHCR.