GENEVA, Dec. 28 (Xinhua) -- At least 17 people were killed and about 140 missing after the boats in which they were fleeing Somalia capsized off the coast of Yemen, the UN Refugee Agency (UNHCR) said on Thursday.

    The incident, involving four boats smuggling 515 people, occurred late Wednesday as the vessels were spotted approaching the coastline by Yemeni authorities, the agency said in a statement.

    Many of the survivors said they were fleeing the conflict between the Ethiopian-backed Somali government and rival Islamists, and the boats carried mainly Somalis and Ethiopians.

    There was an exchange of fire between Yemeni security forces and the smugglers, and two of the four boats overturned as they tried to escape.

    Yemeni authorities said Thursday they had captured all 17 smugglers and their four boats, and that a search operation was still underway for survivors.

    Antonio Guterres, UN High Commissioner for Refugees, said he was "deeply saddened by this latest tragedy involving smugglers' boats carrying desperate people across the Gulf of Aden."

    "Despite efforts to halt this horrible trade, brutal smugglers continue prey on the desperation of poor people fleeing persecution and violence and those looking for better economic opportunities elsewhere," he said.

    He also called for a concerted international effort "aimed at addressing root causes, educating would-be migrants and cracking down on the smugglers and traffickers based in Somalia."