"It's not known in which direction the wind took them and their fate is unknown," the website quoted the Yemeni coastguard as saying of the missing vessel, which it said carried 35-40 Ethiopians including women and children.

The U.N. refugee agency said five men among 46 Ethiopian and Somali passengers had survived the tragedy, which witnesses said began when the engine was caught in fishing nets of the boat, which had left Djibouti three hours earlier.

"This caused panic among the passengers who began to move around causing the boat to capsize. The two smugglers, Yemeni men, jumped off the boat. It is not known whether they survived," UNHCR chief spokeswoman Melissa Fleming told Reuters.

"UNHCR staff will be interviewing the survivors tomorrow (Tuesday) to learn more," she said, adding the Geneva-based agency had no word yet on the fate of the second boat.

Mass drownings have been frequent as many African migrants in unseaworthy boats try to reach Yemen, which they see as a gateway to wealthier parts of the Middle East and the West.

"The Gulf of Aden is still used by many migrants and asylum seekers trying to get to Yemen and then further on to Saudi Arabia," Jean-Philippe Chauzy, spokesman of the International Organisation for Migration (IOM), told Reuters.

"We've seen a slight change in the route used by smuggling rings; they are increasingly using Djibouti as a stepping stone on the way to Yemen rather than using Bosasso, Somalia. It is a transit areas for asylum seekers from the Horn of Africa trying to reach Yemen," he said.

More than 74,000 Africans fled to Yemen in 2009, a 50 percent increase over 2008 despite instability in the country, according to figures issued by the UNHCR.

Upheaval and economic strains in the Horn of Africa fuelled a rise in migrants paying smugglers and undertaking the hazardous sea crossing, it said.