U.S. President Donald Trump says he is optimistic about his summit with North Korean leader Kim Jong Un, over U.S. efforts to end the threat of Pyongyang's nuclear weapons arsenal."We both expect a continuation of the progress made at first Summit in Singapore. Denuclearization?" Trump said in a Twitter message Sunday. After their first meeting last June, Trump boasted as he returned to Washington, "Everybody can now feel much safer than the day I took office. There is no longer a Nuclear Threat from North Korea." But as he meets Wednesday and Thursday with Kim in Hanoi, the Vietnamese capital, there is little concrete evidence that progress has been made to set the specific terms of North Korea's promised denuclearization.'No change' Secretary of State Mike Pompeo told CNN on Sunday "there is no change" in U.S. economic sanctions targeting North Korea until it agrees to "full verifiable denuclearization."He said the United States is "happy to make security assurances" for North Korea's survival as an independent state, to "make North Korea more like South Korea" as an economic power. He said the U.S. is offering North Korea an alternative to "becoming a pariah state."But he acknowledged "we've got work to do" to reach an agreement on how and when Pyongyang would destroy its nuclear arsenal."A demonstrable step (toward denuclearization) is very much what President Trump is focused on," Pompeo said. The top U.S. diplomat said American officials are aware of North Korea's history, over decades, of making promises to disarm and then abrogating agreements. Arrivals Trump said he is leaving for Hanoi early Monday, with Kim already headed to Vietnam in an armored train.Kim's green and yellow train was spotted late Saturday crossing into China and media reports said it then headed south, ending speculation Kim might stop in Beijing before going to Hanoi.North Korea's official news media carried photos of Kim boarding his train and announced he was heading to Vietnam to meet with the U.S. president for a second time. It was the first time that North Korean state media have reported on the summit. Economic PowerIn the early months of his presidency, Trump said he would unleash "fire and fury like the world has never seen” on North Korea for its threats against the U.S. and its allies.But on Sunday, Trump tweeted, "Great relationship with Chairman Kim!" The U.S. leader said Kim "realizes, perhaps better than anyone else, that without nuclear weapons, his country could fast become one of the great economic powers anywhere in the World. Because of its location and people (and him), it has more potential for rapid growth than any other nation!"Trump said Chinese President Xi Jinping "has been very helpful in his support of my meeting with Kim Jong Un. The last thing China wants are large scale nuclear weapons right next door. Sanctions placed on the border by China and Russia have been very helpful." U.S. intelligence officials remain skeptical that North Korea intends to follow through on Kim's Singapore pledge to denuclearize.Director of National Intelligence Dan Coats told a congressional panel last month that North Korea "has halted its provocative behavior" by refraining from missile tests and nuclear tests for more than a year. "As well, Kim Jong Un continues to demonstrate openness to the denuclearization of the Korean Peninsula," Coats said. Despite the end to testing, Coats said, "We currently assess that North Korea will seek to retain its (weapons of mass destruction) capabilities, and is unlikely to completely give up its nuclear weapons and production capabilities.""Our assessment is bolstered by our observations of some activity that is inconsistent with full denuclearization," he added.Coats said the North Korean leader and the rest of the country's rulers "view nuclear weapons as critical to regime survival."