WASHINGTON (Reuters) - U.S. national security adviser Robert O’Brien will visit Vietnam this week, an American official and a Vietnamese source said on Tuesday. O’Brien will be in the Southeast Asian country from Friday to Sunday for high-level meetings, said the U.S. official, who did not want to be identified. The White House did not immediately respond to a request for confirmation. The planned trip will follow a visit to Hanoi last month by U.S. Secretary of State Mike Pompeo. Pompeo’s visit came a week after Vietnam freed a Vietnamese-born U.S. citizen, Michael Nguyen, who had been sentenced to 12 years in prison for “attempting to overthrow the state.” [nL1N2HL1BR] Vietnam and the United States have been marking the 25th anniversary of the normalizing of diplomatic ties. The former Vietnam War foes now have warm relations in spite of U.S. concerns about Hanoi’s human right’s practices, and share worries about China’s increasingly assertive behavior in Asia. O’Brien said during a summit meeting with Southeast Asian countries last weekend that he would visit Vietnam soon.