"Our information is that they were captured by Somali pirates," said Sri Lankan navy spokesman Gayan Wickramasuriya, adding that the Lorenzo Putha-4 had been seized on Saturday some 840 nautical miles southeast of the Somali capital Mogadishu.
Yemen's Iran-backed Houthis have launched scores of attacks in the Red Sea and Gulf of Aden targeting Israeli-linked vessels in response to Israel's war against the Palestinian militant group Hamas.
International naval forces have been diverted north from the Gulf of Aden into the Red Sea, sparking fears of resurgent pirates exploiting the gap, with the first successful case of Somali piracy since 2017 recorded in December.
"Sri Lanka has alerted the Combined Maritime Forces," Wickramasuriya told AFP, referring to the international anti-piracy coalition.
He said the navy of neighbouring India had already sent a warship to investigate, while the Sri Lankan navy was also preparing a vessel to travel to the area.
Pirate attacks off the Somali coast peaked in 2011 -- with the gunmen launching attacks as far as 3,655 kilometers (2,270 miles) from the Somali coast in the Indian Ocean -- before falling off sharply in recent years.
Last month Somali pirates hijacked the bulk carrier MV Ruen.
The Bulgaria-owned and Malta-flagged vessel was seized by Somali pirates 380 nautical miles east of the Yemeni island of Socotra on December 16.
The pirates, who released one injured sailor into the care of the Indian navy, took the MV Ruen and its remaining 17 crew members to Somalia's semi-autonomous state of Puntland.
Earlier this month, Sri Lanka announced it was joining a US-led maritime taskforce in the Red Sea to protect international shipping against attacks by Houthi rebels.
However, the cash-strapped island is yet to send a vessel.
President Ranil Wickremesinghe has said that Houthi attacks had raised freight costs and were impacting Sri Lanka's exports of garments and tea.
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