South Korea's ruling and main opposition parties denounced North Korea's firing Monday of artillery near the western sea border that raised tensions on the divided peninsula, a day after Pyongyang threatened a "new form" of nuclear test. North Korea fired artillery shells near the Yellow Sea border, known as the Northern Limit Line (NLL), as part of military exercises, and some of the shells landed in waters south of the sea border, prompting the South's military to fire back toward the North. The North had earlier notified the South of plans for a firing exercise, warning the South to stay away from seven areas north of the NLL that it designated as firing zones. The exchange of fire forced residents on the front-line island of Baengnyeong to evacuate to shelters. The firings came a day after the North threatened a new form of nuclear test. The North did not elaborate, but outside experts suspect that the "new form" of nuclear test could be based on enriched uranium. The North has so far conducted three nuclear tests, all based on plutonium. "We strongly condemn North Korea's provocations that break in a moment efforts to improve relations with the North under any circumstances," said Rep. Min Hyun-joo, spokeswoman of the ruling Saenuri Party. "North Korea's provocations today not only threaten the Korean Peninsula, but also disregard international relations and put peace in Northeast Asia in serious crisis."    Min urged the government to do its best to ensure the safety of its people. The main opposition New Politics Alliance for Democracy (NPAD) also denounced the firings, saying it was "unacceptable under any circumstances" for the North to drop shells in the South's waters and create military tensions even though the communist nation informed the South of its firing exercise plans. NPAD spokesman Park Kwang-on also accused the North of creating tensions with its mention of a new form of nuclear test. He urged the government to "respond sternly to military provocations while at the same time sticking to the principle of maintaining peace on the Korean Peninsula."