US President Barack Obama smiles as he delivers a speech during a joint press conference at the International Convention Center in Hanoi on May 23, 2016. Photo credit: Luong Thai Linh/AFP

President Barack Obama said Monday he remained confident the Trans-Pacific Partnership (TPP) trade deal would be ratified in the United States despite strong political opposition in Washington.

"The reason I remain confident is it's the right thing to do," he told reporters in Vietnam where he is currently on a three-day visit.

"I have not yet seen a credible argument that once we get TPP in place we are going to be worse off... we're going to be better off," he told reporters in a press conference with his Vietnamese counterpart.

But the US leader conceded getting the ambitious tariff-slashing deal through a hostile, Republican-dominated Congress will be "noisy".

Even elements of his Democrat party are opposed to the deal, which aims to gain lower tariff access and bring down trade barriers to US goods in a market representing 40 percent of the global economy.

The pact also aims to wrest influence from a booming China, which dominates Asian trade.

But critics warn TPP will damage American business by giving cheaper overseas goods preferential access to its domestic market, slashing wages and jobs.

Vietnam has readily embraced the deal and on Monday President Tran Dai Quang backed the TPP as a game-changing pact that can reshape global trade.

He said the pact can "be a driver of economic growth in (the) Asia-Pacific region", adding Vietnam "is committed to fully implementing" all of its clauses, which include recognition of workers' rights.

The deal must now be ratified by the 12 participating countries.

The 12 signatories to the TPP agreement are: Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore, the United States and Vietnam.