Nairobi/Khartoum  - Twenty-one refugees are believed to have drowned after an overloaded boat capsized in a Sudanese river, the UN refugee agency (UNHCR) said on Wednesday.

Eyewitnesses told UNHCR that the refugees were part of a larger group that tried to cross the Atbara River, near the Shagarab refugee camp in eastern Sudan, on Tuesday night.

One of four boats, designed to carry 15 passengers but crammed with 26, overturned 600 metres from shore.

Four Eritrean men and one Somali woman survived. The bodies of the other 21 are yet to be recovered.

"Last night's tragic incident highlights the plight of refugees in eastern Sudan and the inherent risks of smuggling people," UNHCR said in statement.

"Poor living conditions ... and the absence of any prospects compel refugees ... to embark on perilous journeys, most often at the mercy of smugglers, in the hope of reaching Khartoum and, ultimately, a European destination," it added.

UNHCR estimates the number of refugees in eastern Sudan - most of them from Eritrea, Ethiopia and Somalia - at around 130 000. (dpa)