* Refugees travelling towards Sudan
* UNHCR confirms deaths
* IOM aware of survivors brought to Hodeidah (Updates with Somalia calling for an investigation)
DUBAI, March 18 (Reuters) - Forty-two Somali refugees were killed when a helicopter gunship attacked their boat off Yemen on Thursday, the United Nations refugee agency said, and Somalia called on the Saudi-led coalition fighting in the country to investigate.
Mohamed al-Alay, a coastguard officer in the Houthi-controlled Hodeidah area, said the refugees, carrying official UNHCR documents, were going to Sudan from Yemen when an Apache helicopter attacked near the strategic Bab al-Mandab strait.
The area is part of a broad front where forces loyal to Yemeni President Abd-Rabbu Mansour Hadi, backed by a Saudi-led coalition of Arab states, are fighting the Iran-allied Houthi movement which controls most of north and western Yemen.
It was not immediately clear who carried out the attack.
The UNHCR said on its Twitter account that 42 refugees were reported dead and 39 wounded were being treated in hospitals. The International Committee of the Red Cross (ICRC) had earlier said 33 were dead, 29 wounded and other passengers were missing.
"We do not know who carried it out but survivors said they came under attack from another boat at 9 p.m., the crew used lights and shouted to signal this is a civilian boat," ICRC spokeswoman Iolanda Jaquemet said.
"Nevertheless, it did not have any effect and a helicopter joined in the attack," she said.
Somalia's Foreign Affairs Minister Abdusalam Hadliye Omar expressed sorrow at what he described as a "horrific attack".
"It is very sad to target a boat carrying Somali migrants near the coast of Hodeidah in Yemen," he said in a statement. "We call on our partners in the Saudi-led coalition to investigate the raid."
The Saudi-led coalition said it did not conduct any operations or have any engagement on Thursday in the Hodeidah area where the attack took place.
Coalition spokesman General Ahmed al-Asseri said Hodeidah remained under the control of the Houthis and the port continued to be used for "trafficking people, smuggling weapons and attacks against the line of communications in the Red Sea".
Shabia Mantoo, the UNHCR spokeswoman in Yemen, said the vessel was carrying 140 people and trying to sail north out of Yemen when it was hit. She said many of the estimated 255,000 Somali refugees in Yemen are trying to leave because of the war.
"We are distressed by this incident and understand that refugees were travelling in a vessel off the coast of Hodeidah which was reportedly impacted during the course of hostilities," she said.
A sailor who had been operating the boat, Ibrahim Ali Zeyad, said 80 refugees had been rescued.
"We just got information of a helicopter assault on a boat leaving Yemen, we believe for Sudan, full of Somalis," International Organization for Migration (IOM) spokesman Joel Millman told a news briefing in Geneva, adding he was aware of 80 survivors being brought to hospitals in Hodeidah.
ICRC's Eric Christopher Wyss said: "It was a heartbreaking scene. I saw many men, women and children either killed or horribly wounded."
The Saudi-led coalition was formed in 2015 to fight the Houthis and troops loyal to former president Ali Abdullah Saleh who have fired missiles into neighbouring Saudi Arabia.
The Bab al-Mandab is a strategic waterway in the Red Sea through which nearly 4 million barrels of oil are shipped daily. (Additional reporting by Sami Aboudi in Dubai, Stephanie Nebehay in Geneva and Abdi Sheikh in Mogadishu,; writing by Maha El Dahan and William Maclean; editing by Paul Tait and David Clarke)
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