Trade, security and human rights will top the agenda when US President Barack Obama visits Vietnam later this month, a State Department official said Tuesday, adding there was ‘no decision’ yet on lifting an arms embargo to its former wartime foe.

The visit comes as Obama tries to seal his flagship, but all too often distracted, ‘pivot’ to Asia, hoping to bolster Washington's influence in a region where China's growing clout has rattled many neighbours.

Speculation has swirled that the centrepiece of the visit, the exact dates for which have yet to be confirmed by the White House, could be the complete rollback of a decades-old US arms embargo on Vietnam.

Washington partially lifted a 40-year ban on arms sales to Vietnam in 2014, but Hanoi is desperate for modern military hardware as a reward for joining trade pacts with the US and as it looks to confront Beijing's militarisation of the disputed South China Sea.

But US Assistant Secretary of State for East Asian and Pacific Affairs Daniel Russel on Tuesday said the move was far from settled, linking the rights environment in authoritarian Vietnam to any arms deal.

‘No decision has been made,’ on dropping the embargo, Russel said during a visit to Hanoi to pave the way for the president.

‘One of the important factors that would make a lift of the ban possible would be continued forward momentum in meeting universal human rights standards and progress in important legal reforms.’

Vietnam still ruthlessly cracks down on protests, jails dissidents and bans trade unions.

A group of dissidents met Assistant Secretary of State Tom Malinowski in Hanoi on Tuesday, but authorities stopped some from attending, activists said.

One of the dissidents -- the pop singer Mai Khoi, who was recently disqualified from standing in national elections alongside dozens of other activists -- called on Obama to push for further reform.

‘I hope before signing anything with the Vietnamese authorities, the president will create some pressure so that the Vietnamese administration can change,’ she told AFP.

- Contested seas -

Vietnam is also locked in a bitter struggle with Beijing for control of areas of the South China Sea.

Hanoi accuses Beijing of militarising disputed reefs and atolls and is desperate to bolster its naval power to deter its giant neighbour. 

The contested seas are good fishing grounds, a major international shipping route and are believed to contain significant oil and gas reserves.

Washington views the South China Sea as a key trade route and Obama's administration has cosied up to many of China's rival claimants -- including Vietnam.

While the US insists it does not take sides on ownership of the waters, Obama is making several hugely symbolic visits to Southeast Asia in the twilight of his presidency.

Trade will also loom large during Obama's visit, Russel added, focusing on implementing the Trans Pacific Partnership free trade agreement between the US and Asian countries.

Once ratified by the 12 participating countries, its backers say the deal will slash tariffs and trade barriers for an enormous 40 percent of the global economy and establish the largest free trade area in the world.

Relations between Vietnam and the US have improved markedly in the last two decades.

In 2000, Bill Clinton became the first sitting president to visit the former wartime foe since the end of hostilities in 1975.

George W. Bush visited during a regional summit in 2006 but his trip was noticeably more low key than his predecessor.