Families of eight Sri Lankan crew members held captive by suspected Somali pirates on oil tanker Aris 13 in the Indian Ocean pleaded Wednesday (15 March) for the men to be released unharmed after the hijackers demanded a ransom.

In the first hijack of a commercial ship by Somali pirates since 2012, the freighter turned off its tracking system and diverted its course towards the Somali coast after sending a distress call, John Steed of aid group Oceans Beyond Piracy was quoted as saying by Reuters.

Eight crew members, reportedly on board, are still held on the vessel. The hijackers demanded a ransom for the ship's release late on Tuesday, the European Union anti-piracy naval force confirmed.

The hijackers are yet to give any details about the size of the ransom.

Namali Makalandawa, sister of the oil tanker's chief officer Premnath Ruwan Sampath, was quoted as saying by AP news agency that families had tried to contact the shipping company's office in the United Arab Emirates but their calls were not answered.

The Aris 13 is owned by Panama company Armi Shipping, managed by Aurora Ship Management in Dubai.

"Some fear is developing in our hearts. We fear for the lives of our loved ones," Makalandawa is reported as saying after she and other families met with Sri Lankan foreign ministry officials.

Makalandawa revealed that the families have no way of communicating with the captive crew. "Please release them. I appeal to you because these crew members include fathers, sons and husbands. The have gone to sea to earn money to sustain their families," she said.

The European Union anti-piracy naval force confirmed it was contacted the ship's master who said his vessel and crew were being held captive anchored off the coast, near a town called Alula (Caluula), in north-east Somalia.

Britain's foreign secretary Boris Johnson, meanwhile, made a surprise visit to Horn of Africa nation on Wednesday for talks with the country's new president, Mohamed Abdullahi "Farmajo" Mohamed.