KHARTOUM, Sept 24 (Reuters) - More than 20 refugees from Eritrea and Somalia are feared to have drowned in eastern Sudan after an overloaded boat that was smuggling them across a river capsized, the U.N. refugee agency said on Wednesday.

Four Eritrean men managed to swim to the bank of the Atbara river and a Somali woman survived by clinging to a floating log, but 21 others including eight women and three children were missing since Tuesday morning, the UNHCR said.

"There were just too many people on the boat," a UNHCR spokeswoman told Reuters. "Eyewitnesses said the boat was already full when another group came and just jumped in."

One survivor told agency staff the refugees paid $100 each for smugglers to take them from Shagarab refugee camp in eastern Sudan's Kassala state to the capital Khartoum, where they hoped to find work.

They made the river crossing with three other boatloads of refugees, to get round a Sudanese government road block, said the agency.

"Poor living conditions and the absence of any prospects compel refugees ... to embark on perilous journeys in the hope of reaching Khartoum and, ultimately, a European destination," the spokeswoman said.

The UNHCR says there are about 130,000 refugees in Sudan, mostly from Eritrea, Ethiopia, and Somalia, most of them living in camps. Hundreds of thousands of Sudanese have also been displaced by decades of internal conflicts. (Writing by Andrew Heavens; Editing by Matthew Tostevin)

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