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North Korea made a rare expression of gratitude Friday after South Korea sent swine flu medicine, an official said, a day after Pyongyang threatened retaliation over naval drills near their disputed sea border. A convoy of South Korean trucks crossed the heavily armed border into North Korea earlier in the day to deliver enough doses of the antiviral drugs Tamiflu and Relenza for 500,000 North Koreans. The shipment marks the South Korean government's first humanitarian aid since conservative President Lee Myung-bak took office in early 2008 with a pledge to pursue a hard-line policy toward the North and hold it accountable to its nuclear disarmament pledges. South Korean medical officials explained to North Korean counterparts how to use the medicine and its side effects, Kim Young-il, the head of the South Korean delegation, said after returning from the North's border city Kaesong. "We express thanks to South Korea for sending medicine," Kim quoted North Korean Health Ministry official Han Su Chol as saying. North Korea acknowledged for the first time last week that swine flu had broken out in the country after Seoul offered unconditional aid to help contain its spread. The North did not mention any virus-related deaths, but a Seoul-based civic group said the disease killed about 50 people in the North since early November. The delivery came a day after North Korea threatened retaliation over what it claimed were South Korean naval drills around their disputed sea border, accusing Seoul of attempting to escalate tension. On Thursday, the North's Korean Central News Agency cited an unidentified source as saying the South Korean military staged underwater explosive exercises around the border - the scene of a naval clash last month that left one North Korean sailor dead and three others wounded. The drills represent "a threat and an unpardonable act of crime against us," KCNA said. "We cannot but view this as a premeditated provocation aimed at raising tension in the militarily sensitive waters," it said. "We will deal a merciless retaliatory blow if the South Korean warlike forces keep staging military provocations." South Korea's Joint Chiefs of Staff dismissed the North's claim, saying Friday the drills were routine and took place in the South's waters. North Korea has often made such accusations and the South has rejected them. Pyongyang, known for its use of fiery rhetoric, also regularly threatens South Korea with destruction. The communist North does not recognize the sea boundary, drawn by the United Nations at the end of the 1950-53 Korean War, and has long claimed it should be redrawn farther south. The dispute led to deadly skirmishes in 1999, 2002 and last month. Relations between the two Koreas soured badly after Lee halted unconditional aid to the North in line with his pledge to get tough on the communist neighbor over its nuclear weapons development. The two sides are still technically at war as the Korean War ended in a truce, not a peace treaty.