Their comments came shortly after Vietnam, the US and six other countries concluded the second round of talks toward a proposed Trans-Pacific Partnership (TPP) trade deal.

"We wanted to explain to all the government officials we met with that the issue of labour rights, worker rights, freedom of organisation is an essential an essential part of any future liberalisation of trade agreements with the United States," Senator Tom Harkin told reporters.

The Iowa Democrat is chair of a Senate committee responsible for labour and other issues.

Vietnam bans labour unions that are independent of the ruling Communist Party.

US-based Human Rights Watch said in May that Vietnamese authorities had re-arrested Doan Huy Chuong, 25, one of the founders of an independent trade union, and held him almost completely incommunicado.

Harkin, along with two other members of his committee - Senators Al Franken of Minnesota and Bernie Sanders of Vermont - was to meet prime minister Nguyen Tan Dung later Wednesday.

They earlier held talks with officials including the minister of Labour Nguyen Thi Kim Ngan.

Sanders told reporters that before any free-trade agreements could be established it was imperative that "workers in Vietnam have the right to organise, to form unions, to negotiate contracts... independent unions elected by the workers themselves."

Labour issues are among those being discussed as part of the prospective TPP deal. The eight participants held a second round of talks last month in the US.

The Senate delegation to Vietnam comes 15 years after the normalisation of diplomatic relations with the US, following a war that lasted several years.

Despite differences over human rights issues, relations between the two countries have grown across a range of areas, and the US is Vietnam's most important export market.

Agence France Presse - July 9, 2010