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SEOUL, South Korea (AP) — South Korea’s prime minister on Tuesday apologized for “failing to carefully take care of the health” of hundreds of sailors who contracted the coronavirus on a navy ship taking part in an anti-piracy mission off East Africa.

The outbreak aboard the destroyer Munmu the Great is the largest cluster South Korea’s military has seen. A total of 247 of the ship’s 301 crew have tested positive for COVID-19 in recent days, and the entire crew returned to South Korea aboard military planes on Tuesday evening.

None of the crew had been vaccinated because they left South Korea in early February, before the start of the country’s vaccination campaign.

Prime Minister Kim Boo-kyum said in televised comments that the government is “very sorry for failing to carefully take care of the health of our soldiers who are devoting themselves to the country.”

In a separate address, Defense Minister Suh Wook said he “feels heavy responsibility for (the outbreak) and offers words of sincere apology” to the sailors, their families and the public. He said the government will work out measures to prevent similar outbreaks involving South Korean troops dispatched abroad.

After arriving at a military airport near Seoul, the 301 sailors were moved to hospitals or quarantine facilities depending on their condition. Media photos showed masked sailors waving their hands through the windows as their buses left the airport. Health officials said all of the sailors would be tested again in South Korea.

The cause of infections on the 4,400-ton-class destroyer hasn’t been announced. But military authorities earlier suspected the outbreak might have begun when the destroyer docked at a harbor in the region to load goods in late June.

South Korea has been engaging in anti-piracy operations in the Gulf of Aden since 2009.

The military planes dispatched to bring back the sailors carried other navy personnel who are to sail the Munmu the Great back to South Korea in a journey expected to take one month. Another South Korean destroyer was on its way to the area to replace the Munmu the Great, according to the Joint Chiefs of Staff.

The outbreak on the destroyer comes as South Korea is battling its worst surge in the pandemic at home.

On Tuesday, South Korea reported 1,278 new virus cases. It was the 14th day in a row that it has reported more than 1,000 new cases.

Since the pandemic began, South Korea has reported 180,481 infections and 2,059 deaths.