The International Organization for Migration has decried the apparent helicopter attack on a boat off the Yemeni coast that killed dozens of Somali refugees, calling it "unacceptable".

Officials in the semi-autonomous region of Puntland in northeast Somalia told Al Jazeera the boat was heading from Hodeidah to Sudan, a major transit point for refugees and migrants to Europe.

Mohammed Abdiker, emergencies director of the Geneva-based IOM, said the civilian casualties could have been avoided in Friday's attack if "they could have checked to know who was in that boat before firing on it".

The International Committee of the Red Cross (ICRC) put the death toll at 33, adding that 29 people were wounded and some other passengers were missing.

It was not immediately clear who carried out Friday's attack.

"They shot at us, we could see the flashes of bullets. We were shouting at them to hear us. Signalling with lights, but they kept shooting. The Apache came above us and shot at us. Our friends were dying," Al-Hassan Ghaleb Mohammed, a survivor, told Al Jazeera.

Survivors conveyed conflicting accounts of who was behind the attack.

"We do not know who carried it out, but survivors said they came under attack from another boat at 9pm, the crew used lights and shouted to signal this is a civilian boat," Iolanda Jaguemet, ICRC spokesperson, told Reuters news agency.

"Nevertheless, it did not have any effect and a helicopter joined in the attack."

Arab coalition statement

An Arab coalition that is involved in fighting in Yemen said it did not conduct any operations or have any engagement in the Hodeidah area.

The coalition was assembled by Saudi Arabia in 2015 to fight the Houthis and troops loyal to Ali Abdullah Saleh, the former long-serving president, who have fired missiles into neighbouring Saudi Arabia.

General Ahmed Asseri, the Arab coalition spokesman, 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".

The UNHCR says more than 250,000 Somali refugees are in Yemen.

Hodeidah, on the Red Sea, is controlled by the Houthi fighters, who in 2014 overran Yemen's capital Sanaa and forced the government of President Abd-Rabbu Mansour Hadi to flee into exile.