Kerry, a practicing Catholic whose experiences during the Vietnam War inspired his political activism, visited the French-colonial era Notre Dame Cathedral in southern Ho Chi Minh City as he began his first official visit to the nation as the top US diplomat.
Washington is eager to underscore its commitment to Asia after its "pivot" policy was shaken earlier this year when the US government shutdown forced President Barak Obama to cancel a trip to the region, allowing China to occupy centre stage at key regional summits.
The region is beset by political and territorial tensions, including bitter maritime disputes between an increasingly assertive Beijing and a number of its neighbours, among them Vietnam, that have raised concerns a minor incident in contested waters could escalate rapidly.
Kerry is scheduled to meet Prime Minister Nguyen Tan Dung and Foreign Minister Pham Binh Minh in Hanoi as part of his three-day visit, and is expected to highlight growing security and trade ties between Washington and Hanoi, which normalised relations two decades ago.
He is also due to visit to the Mekong Delta where he served as a naval lieutenant.
During the war he served with the US Navy from 1966 to 1970 and was a Swift Boat skipper on the dangerous gunboat missions patrolling the rivers of the Mekong Delta.
Kerry, who celebrated his 70th birthday on Wednesday, has voiced excitement over his trip, saying he has not gone to Vietnam since he joined president Bill Clinton on his landmark visit in 2000.
Washington has sought to bolster its footprint in Asia as part of efforts to balance the growing might of China in the region.
The Philippines, Vietnam, Malaysia and Brunei all have their own disputes with China over territory in the South China Sea, while Beijing has also locked horns with Tokyo over areas in the East Sea.
Kerry's trip will also include a visit to the Philippines.
It is a chance to "tighten the slack" left by Obama's absence at the APEC forum in Bali this year, said Jonathan London of the Department of Asian and International Studies at City University of Hong Kong.
"Kerry's visit is an opportunity for (Vietnam and the US) to more clearly define ways forward in the context of an East Asian diplomatic scene that has been destabilised by China's increasing aggressive regional posture," he told AFP.
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