North Korea on Saturday renewed threats to retaliate if South Korean forces carry out a planned live-fire drill near the countries' disputed maritime border, according to dpa. The North Korean military would fight back "mercilessly" if the Yellow Sea border is breached, the Foreign Ministry was quoted as saying by state media. "Our military does not make empty talk," it added. Seoul announced Thursday that it would hold a day of artillery exercises some time by the following Tuesday on Yeonpyeong island, which recently came under attack from the North. Pyongyang had said on Friday that its retaliation would be "more serious" than on November 23, when it shelled the island, killing two soldiers and two civilians and injuring 18 others. The Foreign Ministry on Saturday also criticised the United States for wanting to deploy about 20 of its soldiers to Yeonpyeong, in support of the maritime drill. The ministry said the soldiers were being used as "human shields." The US currently has some 28,500 troops stationed in South Korea to deter the communist North. It remained unclear Saturday on what day the manoeuvres would take place. Military sources said the timing depends on weather conditions.