Somali pirates killed five crew members of the hijacked MT Honour 25 petroleum products tanker before it was rescued by security forces, Somalia's semi-autonomous state of Puntland said on Tuesday.
Puntland's authorities said they also rescued a second vessel on Tuesday that had been hijacked in May.
The deaths aboard the Palau-flagged ship appear to be the first at the hands of Somali pirates in over a decade.
The ship with 17 crew members was seized by pirates in April during a resurgence of Somali piracy that experts have attributed, in part, to impacts of the Iran war, including elevated oil prices that make tankers more attractive targets.
Rescue mission successfully freed ship, Puntland's information ministry says
Puntland's information ministry said in a statement that a rescue mission had successfully freed the ship and captured 16 pirates. "Before the operation began, the pirates on board the ship killed five of the crew. Among those killed were three from Pakistan, one from Myanmar and one from India," it said.
Four crew members - three Pakistanis and one Indian - were also injured, the statement said.
The rescue follows another rescue mission last week, when Puntland's maritime police force freed the US-sanctioned tanker Sibu 1, which Somali pirates had hijacked in August.
Puntland's Maritime Police Force (PMPF) said in a statement late on Tuesday that it had also rescued an oil tanker called MT Eureka which had been hijacked in May after its hijackers fled when police approached the vessel.
"As PMPF forces approached the area, the pirates fled towards the shore and moved inland," it said.
"PMPF forces continue to pursue the individuals involved and are conducting further operations to apprehend those who escaped."
The two rescues bring the total to five out of the six ships held by pirates off the Somali coast as of late August that have now been freed.
A previous wave of piracy from 2008 to 2014 disrupted one of the world's busiest shipping routes, cost the global economy billions of dollars, and resulted in hundreds of millions of dollars in ransom payments.