US President Barack Obama said several Vietnamese civil society members were prevented from meeting him on Tuesday and that, despite great strides made by the country, Washington had concerns about the limits it puts on political freedom.
Obama was due to lay out more of his plan for stronger ties with Vietnam on the second day of his visit, after scrapping an arms ban, the last big hurdle between two countries drawn together by concern over China's military buildup.
The removal of the arms embargo, a vestige of the Vietnam War, suggests U.S. worries about Beijing's building of man-made island in the South China Sea and deployment of advanced radars and missile batteries in the disputed region trumped concern about Vietnam's human rights record.
Washington had for years said a lifting of the ban would require concrete steps by Vietnam in allowing freedom of speech, worship and assembly and releasing political prisoners.
Obama met about six activists and said there were "significant areas of concern" about political freedom. He praised those Vietnamese who were "willing to make their voices heard".
Two activists who spoke to Reuters said an intellectual, Nguyen Quang A, had been taken away by unknown men before he had hoped to met Obama, citing his relatives.
Reuters could not verify the information and Vietnam's foreign ministry did not immediately respond to a request for comment.
On Monday, in a joint news conference with Vietnamese President Tran Dai Quang, Obama said "modest" human rights improvements had been made and the decision to end the arms embargo was about the changing dynamic in ties and "not based on China".
But China's Global Times tabloid, run by the Chinese Communist Party's official People's Daily, said that was a lie and made a point of what it said was a U.S. willingness to relax standards on human rights for the sake of containing China.
The White House "is taking advantage of Vietnam to stir up more troubles in the South China Sea", it said.
Mai Khoi, a Vietnamese singer, was one of the people who met Obama and she posted a photo on her Facebook page showing several people attended the meeting.
Obama was flanked by activists on either side at a table. They listened intently as he spoke at the end of the meeting.
Some activists have expressed disappointment that Obama may have given away leverage with the communist leadership.
A U.S. official, speaking on condition of anonymity, said there was evidence engagement had worked in nudging Vietnam to make concessions, like its "unprecedented" commitment to set up independent labour unions under a U.S.-inspired Trans-Pacific Partnership (TPP) trade deal.
In a statement late on Monday, Communist Party chief Nguyen Phu Trong spoke of the importance of building relations of mutual respect while "not interfering in each other's internal affairs".
Obama gave a speech in Hanoi about the development of relations since normalisation in 1995 and will champion his signature TPP, which would remove tariffs within a 12-nation bloc worth a combined $28 trillion of gross domestic product.
Vietnam's manufacturing and export-led economy is seen as the biggest TPP beneficiary. Annual U.S-Vietnam trade has swelled from $450 million when ties were normalised to $45 billion last year, and Washington is a big buyer of Vietnam's televisions, smartphones, clothing and seafood.
The TPP is not a done deal, with opposition expected in Washington amid concern about competition and a loss of U.S. jobs. Obama said he was confident the trade pact would be approved by legislators and he had not seen a credible argument that the deal would dent American business.
Obama will on Tuesday fly to Ho Chi Minh City, the country's commercial hub, which was called Saigon until North Vietnamese tanks rolled into the city in April 1975 to bring U.S.-backed South Vietnam under communist rule.
He will meet young entrepreneurs at one of the co-working spaces that host Vietnam's budget tech startups, which have been receiving attention from angel investors and Silicon Valley funds.
Obama spoke of a U.S. intention to work more closely in defence areas with Vietnam, which is keen to build a deterrent against China. Vietnam and the United States last year held coastguard and humanitarian training exercises.
Washington has longstanding defence alliances in the region with the Philippines, which is also at odds with China, and Thailand, and organises annual war games with both.
U.S. Secretary of State John Kerry attended a ceremony on Tuesday in which a deal was agreed with Vietnam to allow the U.S. Peace Corps to work there.
Tuesday's English-language China Daily said Obama's visit "bodes ill for regional peace and stability", and would further complicate the situation in the South China Sea, and risk turning the region into a "tinderbox of conflicts".