Forty-four years after first setting foot in the country, as a young naval officer, John Kerry returned to Vietnam on Saturday, this time as America's secretary of state, offering security assurances and seeking to promote democratic and economic reform.

Making his 14th trip to the south-east Asian nation since the end of the war that profoundly influenced his political career and foreign policy thinking, Kerry is trying to bolster the remarkable rapprochement with a former US enemy that he encouraged and helped to engineer as a senator in the 1990s.

"I can't think of two countries that have worked harder, done more and done better to try to bring themselves together and change history, to change the future, to provide a future for people that is very, very different," Kerry told a group of businesspeople, students and others at the US consulate's American Center in Ho Chi Minh City.

The visit is Kerry's first to Vietnam since 2000, when he was part of then-President Bill Clinton's historic trip here, the first by an American president since the end of the war in 1975 and the start of a US embargo. Between 1991 and 2000, Kerry traveled 13 times to Vietnam, beginning with visits to clear up lingering questions over the fate of American prisoners of war and those listed as missing in action from the conflict.

Here, in the city he first knew as Saigon, the capital of the former South Vietnam, Kerry met Saturday with members of the business community and entrepreneurs to talk up the Trans-Pacific Partnership, a wide-ranging trade agreement that the US is now negotiating with Vietnam and nine other Asian countries. To take full advantage of the economic opportunities offered by the pact, Kerry said Vietnam, which has been widely criticised for its human rights record, must embrace changes.

"A commitment to an open internet, to a more open society, to the rights of people to be able to exchange their ideas, to high quality education, to a business environment that supports innovative companies and to the protection of individual people's human rights and their ability to be able to join together and express their ideas, all of these things create a more vibrant and a more powerful economy, as well as a society," Kerry said.

"It strengthens a country, it doesn't weaken it," Kerry said. "The United States urges leaders here to embrace that possibility and to protect those rights."

He made the comments after attending mass at Ho Chi Minh City's Notre Dame Cathedral, which was built in the 1880s and 90s under French colonial rule, in an attempt to show support for the tenuous freedom of worship in Vietnam. Vietnamese authorities have been criticised for harassing, prosecuting and jailing Catholic clergy.

On Sunday, Kerry will travel to the Mekong River delta region, where he was commander of a swift patrol boat in 1968 and 1969. Kerry plans a riverboat cruise to inspect agriculture projects that are a mainstay of southern Vietnam's economy and assess the impact of upstream development and climate change. He will then visit the capital, Hanoi, for talks with senior Vietnamese officials.

US officials with Kerry said he would make a strong case to the Vietnamese that respect for human rights, particularly freedom of speech and religion, is key to improved relations with the US. The officials said they expected Kerry to raise specific cases of political prisoners the US would like to see released.

Apart from human rights and economic reform, the discussions in Hanoi are expected to focus largely on maritime security and territorial disputes in the South China Sea. Vietnam and other members of the Association of Southeast Asian Nations are deeply concerned about China's growing assertiveness in the region and are looking to the US to serve as a counterbalance by stepping up its traditional role as a guarantor of security in the Asia-Pacific.

The Obama administration has pledged to do so as part of its self-described "pivot to Asia", with calls for a binding code of conduct on the high seas to ratchet down tensions between China and its smaller neighbors over disputed territory.

China, however, has reacted angrily to the US approach and earlier this month, over strenuous objections from Washington, announced a new air defense zone over parts of the East China Sea, where it has competing claims with Japan. Chinese officials have since said they might declare a similar zone in the South China Sea.

From Vietnam, Kerry will travel to the Philippines, which has its own maritime disputes with China.