U.S. Democratic presidential contender Hillary Clinton has come out against the recently signed Trans-Pacific Partnership, saying there are too many "unanswered questions" about the comprehensive trade deal.

"What I know about it, as of today, I am not in favour" of TPP, Clinton said during an interview with PBS anchor Judy Woodruff set to air on Wednesday.

"I don't believe it's going to meet the high bar I have set," she said of the deal, the origins of which she helped work on as U.S. President Barack Obama's secretary of state a half-decade ago.

She joins Democratic rivals Bernie Sanders and Martin O'Malley who warn it could lead to lost American jobs. The free trade deal backed by the administration has been opposed by liberal Democrats and labour unions.

In her 2014 book, Hard Choices, Clinton championed the TPP, but said her final approval would depend on the final terms.

"It's safe to say the TPP won't be perfect — no deal negotiated among a dozen countries ever will be — but its higher standards, if implemented and enforced, should benefit American businesses and workers," she wrote, adding that her goal was higher labour and environmental standards and intellectual property protection in member countries.

It has also emerged as a campaign issue in Canada, with New Democrats saying they oppose the deal Canada is a signatory to, and the Liberals saying they want to see more details of what's been agreed on.

Congressional challenge

The pact is central to Obama's attempt to engage with Asia to offset a rising Chinese influence in the region.

But he faces a challenge getting it through Congress. Republican lawmakers passed fast-track legislation earlier this year to ensure a speedy vote on TPP, but some GOP lawmakers may withhold their votes if they don't like the details of the final deal.

Free-trade pacts are unpopular with blue-collar voters, who believe their jobs are being shipped offshore.

When they were campaigning for the presidential nomination in 2008, both Clinton and Obama said they wanted to renegotiate the North American Free Trade Agreement between the U.S., Canada and Mexico.

"We will opt out of NAFTA unless we renegotiate it," Clinton said at the time. "And we renegotiate it on terms that are favourable to all of America."