The U.N. refugee agency says at least 33 migrants trying to reach Yemen have drowned in the Gulf of Aden.

The agency says the drownings occurred after human smugglers forced all 137 passengers off a boat and into deep waters off the Yemeni coast.

It says the boat had sailed from Somalia with 134 Somalis and three Ethiopians on board.  Some of the migrants reached shore, but others are still missing.

The agency says it learned of the tragedy on Saturday, but it was unclear when the incident actually occurred.

Thousands of Somalis and Ethiopians try to cross the Gulf of Aden each year in hopes of escaping poverty and violence in their homelands.

The U.N. says many of the smugglers are ruthless, beating the migrants or making them swim long distances to shore.