HANOI, VIETNAM: US President Barack Obama started his first visit to Vietnam on Monday looking to bolster the government with trade opportunities and the possible lifting of an arms export embargo even as he pushes for better human rights from the one-party state.

He wants to strike this balance with a country Washington sees as a crucial, though flawed partner amid Chinese efforts to strengthen claims to disputed territory in the South China Sea, one of the world's most important waterways.

Lifting the arms embargo would be a psychological boost for Vietnam's leaders as they look to counter an increasingly aggressive China, but there may not be a big jump in sales. White House officials say the idea is under consideration and will be a focus of talks Monday, but US lawmakers and activists have urged the president to press the communist leadership for greater freedoms before granting it.

Vietnam holds about 100 political prisoners and there have been more detentions this year.

The United States has already partially lifted the embargo, but Vietnam wants full access as it tries to deal with China's assertive land reclamation and military construction in nearby seas.

Vietnam has not bought anything, but is still eager for Washington to remove the remaining restrictions. This would show relations are fully normalised and could open the way to deeper security cooperation.

After three days in Vietnam, Obama heads to Japan for an international summit and a visit to Hiroshima, where he will be the first sitting president to visit the site of the first atomic bomb attack.

He arrived in Hanoi, the capital, late Sunday, making him the third sitting president to visit the country since the end of the war. Four decades after the fall of Saigon, now called Ho Chi Minh City, and two decades after President Bill Clinton restored relations with the nation, Obama is eager to upgrade relations with an emerging power whose rapidly expanding middle class beckons as a promising market for US goods and an offset to China's growing strength.

Obama was greeted Monday by Vietnamese President Tran Dai Quang at the Presidential Palace. Obama congratulated Vietnam for making "extraordinary progress." He said he hopes the visit will show a continued interest in strengthening ties in the years to come.

Obama will make the case for stronger commercial and security ties, including approval of the 12-nation Trans-Pacific Trade agreement that is stalled in Congress and facing strong opposition from the 2016 presidential candidates.

The United States is eager to boost trade with a fast-growing middle class in Vietnam that is expected to double by 2020. That would mean knocking down auto, food and machine tariffs to get more US products into Vietnam.

In Japan, Obama will attend a summit of the Group of Seven industrialised nations, where the uncertain global economy will be a top concern. They'll also grapple with a full array of world challenges, including the fight against the Islamic State group in Iraq and Syria and the refugee crisis in Europe.

Obama will finish his trip in Hiroshima, where the US dropped the atomic bomb that killed 140,000 people, ushering in the nuclear age seven decades ago. Another bomb killed 70,000 in Nagasaki three days later.

It will be a moment to reflect on the devastating costs of war and to try to give new impetus to the call for a nuclear-free world that Obama issued seven years ago in his first year as president. He has faced criticism, however, that his mere presence at the site of the a-bomb explosion could be viewed as apology for an act that many Americans see as justified.

Deputy national security adviser Ben Rhodes said the Vietnam and Japan visits both reflect Obama's world view "that we can move beyond difficult and complicated histories" to find areas of common interest.

"You could not have had a more violent conflict than we had with the Japanese in World War II, as a visit to Hiroshima will certainly mark, but now they are among our closest friends in the world," Rhodes said. "You could not have a more contested, controversial, costly, tragic war than the Vietnam War, and now (Vietnam) is becoming a partner of the United States, an important partner."

Still, concerns about human and political rights shadow the president's stay in Vietnam.