The US Senate has approved legislation key to securing the Trans-Pacific Partnership (TPP) trade deal, which could span 12 countries including Australia, advancing US president Barack Obama's efforts to strengthen US economic ties with Asia.

After a six-week congressional battle including two brushes with failure, some fancy legislative footwork and myriad backroom deals to keep the legislation alive, the Senate voted 60 to 38 to grant Mr Obama the power to negotiate trade deals and send them on a fast track through Congress.

The bill next goes to Mr Obama for his signature.

Australia and the other 11 nations could hold their final ministerial meeting next month to wrap up the deal.

The TPP, potentially a legacy-defining achievement for Mr Obama, would be the biggest free trade agreement in a generation and rank with China's ascension to the world trade stage and the North American Free Trade Agreement liberalising trade between the United States, Canada and Mexico.

But TPP ministers still have tricky issues to resolve, ranging from monopoly periods for next-generation medicines to the treatment of state-owned enterprises.

Some member countries, including Japan and Canada, want to have fast track in place before making final offers on the trade deal, which would cover 40 per cent of the world economy and raise annual global economic output by nearly $US300 billion.

Negotiators say a deal on the TPP could be wrapped up within weeks once countries are sure US lawmakers will not pick the deal apart afterward, which fast track prevents.

Fast track authority will last up to six years

Senators showed slightly less support than in an initial vote a month ago for fast track, which would let lawmakers set negotiating goals for trade deals but restrict them to yes-or-no votes on final agreements.

Fast-track authority lasts for up to six years and would extend to any trade deals negotiated by Mr Obama's successor, who would take office in January 2017.

The vote came as Congress was trying to finish up the four parts of the trade legislation package and send them to Mr Obama: fast-track negotiating authority, aid for workers who lose their jobs as a result of trade, an Africa trade preferences bill and a customs enforcement measure.

Fast track was forced back to the Senate floor after a revolt by Democrats in the House of Representatives resulted in it being split from a companion measure extending a program to help workers hurt by trade.

That bill is expected to pass the Senate shortly and will then return to the House.

Many Democrats who opposed the aid program last week now plan to support it, including House Democratic leader Nancy Pelosi.

The top Democrat on the House Ways and Means Committee, Sander Levin, said he expected the "vast majority" of Democrats to vote "yes."

That would allow both measures to go to Mr Obama for approval this week, before lawmakers go on a week-long break.

The bruising congressional battle has pitted Mr Obama against many in his own party, including Ms Pelosi, and prompted blood-letting among Republicans after party leaders lashed out at conservatives who refused to back the trade agenda.

Opinion polls show a majority of Americans support trade in general, but congressional approval has been a slog because labour unions and activists have campaigned against fast track, warning of job losses and vowing to retaliate against Democrats who break ranks to support trade.

The front runner for the party's presidential nomination in 2016, Hillary Clinton, said Democratic critics had legitimate concerns but has so far reserved judgment on the TPP.

Reuters