South Korea on Monday expanded the search for a missing fisheries official killed by North Korean troops at sea last week, a day after North Korea accused the South of raising tension by intruding into its territorial waters.

South Korea’s military has accused North Korean soldiers of killing the man, dousing his body in fuel and setting it on fire near the sea border, apparently in an effort to prevent the risk of a novel coronanvirus outbreak.

Officials in Seoul are calling on North Korea to agree to a joint investigation into the incident, which prompted an apology from North Korean leader Kim Jong Un, who said the killing should not have happened.

South Korea’s President Moon Jae-in said on Monday that military hotlines with North Korea should be restored to prevent unexpected incidents. North Korea severed the inter-Korean hotlines this year as relations soured.

Moon called Kim’s apology “unprecedented, very rare and special” and a sign that North Korea did not want relations to worsen. He added that communication must resume to prevent future problems.

As of Monday, the North had not responded to the call for a joint investigation. On Sunday, its state media issued a statement complaining that South Korea’s naval operations had entered its territorial waters in the area, off the west coast of the peninsula, threatening to raise tensions.

“We have never crossed the Northern Limit Line to the North’s side, but there has been differences in how the two Koreas mark the waters,” South Korea Coast Guard Lieutenant Lee Hong-chear said, referring to a disputed maritime demarcation that dates to the end of the 1950-1953 Korean War.

At least six aircraft and 45 vessels were participating in the search, including 36 ships from the coast guard and navy, and nine boats from the fisheries ministry and private owners, Lee said.

North Korea said on Sunday it was conducting its own search for the man’s body, and said it was considering ways to hand it over to the South if found.