The UN Refugee Agency (UNHCR) said on Wednesday that 21 refugees were feared to have been drowned on Tuesday night when their overloaded boat capsized in the Atbara River, near the Shagarab refugee camp in eastern Sudan.
The corpses are yet to be recovered.
UNHCR said in a statement issued in Nairobi that local administration and security officials, along with UNHCR and its Sudanese Government counterpart, COR, rushed to the scene.
According to eyewitnesses, the refugees were part of a larger group that tried to cross the river in four boats.
"One of four boats meant to carry 15 passengers at most, but packed with 26, capsized some 600-700 metres from shore," the UN agency said.
"Four Eritrean men survived by swimming to shore along with a Somali woman who clung to a floating log. Survivors were transferred to a health clinic at the Shagarab II camp."
The five survivors carried refugee identity cards. A male who said he arrived in Shagarab camp three weeks ago, was travelling to Khartoum in search of work. He and several others were offered the trip across the river and onward to Khartoum by road for a fee of about 100 dollars each.
"Among the missing were 11 Eritrean and Somali families, including eight women at least three children, according to eyewitness. Two suspected smugglers -- also refugees -- are now in police custody," UNHCR said.
Meanwhile, UNHCR said arrangements have been made for a dignified burial of the bodies, if and when they were recovered.

"Last night's tragic incident highlights the plight of refugees in eastern Sudan and the inherent risks of smuggling people. The boat crossing was intended to bypass road blocks out of the camp as government regulations stipulate that refugees must remain in camps and receive assistance there," it said.
Poor living conditions, however, and the absence of any prospects compel refugees, including women and children, to embark on perilous journeys (most often at the mercy of smugglers) in the hope of reaching Khartoum and, ultimately, a European destination.
UNHCR estimates the number of refugees in eastern Sudan at around 130,000. They originate mostly from Eritrea, Ethiopia, and Somalia.
Close to 100,000 live in 12 camps where they receive international assistance while an estimated 40,000 eke out a living in rural and urban concentrations.