UNITED NATIONS, Sept 21 (Reuters) - South Korea President Moon Jae-in on Tuesday addressed the U.N. General Assembly and repeated a call for a declaration to formally end the 1950-1953 Korean War. "I once again urge the community of nations to mobilize its strengths for the end-of-war declaration on the Korean Peninsula," Moon said in a speech to the annual gathering of the world body. "I propose that three parties of the two Koreas and the U.S., or four parties of the two Koreas, the U.S. and China come together and declare that the War on the Korean Peninsula is over," he said. North Korea had long sought a formal end to the Korean War to replace the armistice that stopped the fighting but left it and the U.S.-led U.N. Command still technically at war. Moon, who has been active in trying to engage with North Korea throughout his presidency, has argued that such a declaration would encourage North Korea to give up to denuclearize. Washington has said Pyongyang must give up its nuclear weapons first. Earlier on Tuesday, U.S. President Joe Biden addressed the U.N. assembly and said the United States sought "serious and sustained diplomacy to pursue the complete denuclearization of the Korean Peninsula." "We seek concrete progress toward an available plan with tangible commitments that would increase stability on the Peninsula and in the region, as well as improve the lives of the people in the Democratic People’s Republic of Korea," he said, using North Korea"s official name. North Korea has brushed off U.S. calls for a return to dialogue and the head of the U.N. atomic watchdog said this week that Pyongyang"s nuclear program is going "full steam ahead." read more