PANMUNJOM, South Korea (Reuters) - U.S. Defense Secretary Jim Mattis emphasized diplomatic efforts to resolve the North Korean missile and nuclear crisis as he stood at the tense and heavily fortified border between North and South on Friday, saying:  Our goal is not war.  His remarks came before U.S. President Donald Trump - who has threatened to destroy the North if necessary - leaves on his first trip to Asia next week, including a stop in South Korea to meet President Moon Jae-in. For his part, Moon, after talks with Mattis, said the  aggressive deployment  of U.S. strategic assets in the region, which have included overflights by U.S. bombers, had been effective in deterring the North Korean threat. Tension between North Korea and the United States has been building after a series of nuclear and missile tests by Pyongyang and bellicose verbal exchanges between Trump and North Korean leader Kim Jong Un, stoking fears any miscalculation could lead to an armed confrontation.   North Korean provocations continue to threaten regional and global security despite unanimous condemnation by the United Nations Security Council,  Mattis said in prepared remarks as he visited the demilitarized zone (DMZ).  As Secretary of State Tillerson has made clear, our goal is not war, but rather the complete, verifiable, and irreversible denuclearization of the Korean peninsula.  Standing alongside Mattis, South Korean Defense Minister Song Young-moo said:  We together will continue to defend peace through strong will and strong might.  Ahead of Trump s visit to Asia, Mattis has emphasized diplomatic efforts to find a peaceful solution to the crisis during his week-long trip to the region.  That s really what it was all about   to keep the (North Korea) effort firmly in the diplomatic lane for resolution,  Mattis said earlier this week after three days of meetings with Asian defense chiefs in the Philippines.  At the same time, the U.S. and South Korean militaries are looking for ways to deter Pyongyang and bolster the South s defenses. Washington s top military officer, Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, met with his South Korean counterpart, General Kyeong Doo Jeong, a U.S. military statement said. Dunford renewed U.S. warnings of retaliation to further provocations.   (Dunford) reaffirmed that any attack by North Korea would be met with a response that will be overwhelming and effective, using the full range of U.S. military capabilities,  the statement said. The United States flew Air Force bombers over waters east of North Korea last month in a show of force. The U.S. Navy, in what it says was a long-planned maneuver, will have three U.S. aircraft carrier strike groups in the Pacific in the coming days.  Last week, CIA chief Mike Pompeo said North Korea could be only months away from developing the ability to hit the United States with nuclear weapons, a scenario Trump has vowed to prevent.  U.S. intelligence experts say Pyongyang believes it needs the weapons to ensure its survival and have been skeptical about diplomatic efforts, focusing on sanctions, to get Pyongyang to denuclearize. The United States on Thursday imposed sanctions on seven North Korean individuals and three entities for  flagrant  human rights abuses, including killings, torture, forced labor and the hunting down of asylum seekers abroad. In a speech last month at the United Nations, Trump threatened to destroy North Korea if necessary to defend the United States and allies. Kim has blasted Trump as  mentally deranged.  Despite the rhetoric, White House officials say Trump is looking for a peaceful resolution. But all options, including military ones, are on the table.   Do we have military options in defense for attack, if our allies are attacked? Of course we do. But everyone is out for a peaceful resolution,  Mattis told reporters traveling with him this week.  No one s rushing for war.  Separately, North Korea released a South Korean fishing boat which had been found to be in North Korean waters illegally, state media said. The crew of 10 - seven South Koreans and three Vietnamese - were released on Friday evening, a spokesman for South Korea s coastguard told Reuters. The return of the boat, which had been reported as missing from Saturday, may have eased already strained relations between North and South. The two sides are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and the United States.   