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US President Donald Trump has said he will only join the Trans Pacific Partnership (TPP) if the deal is "substantially better" than the one offered to President Barack Obama.

The TPP is the free trade pact with primarily Asia-Pacific nations that Mr Trump backed out of last year.

Just hours earlier, Mr Trump had unexpectedly said the US would consider rejoining the pact.

The president has previously criticised the deal as a potential "disaster".

But his trade strategy is under fire as a conflict escalates with China.

It has also upset some of his fellow Republicans - especially those representing farmers who are expected to be hurt should the latest proposed tariffs be pushed through.

Politicians on both sides are worried that Mr Trump is leading the US into a damaging economic battle with China, after levying tariffs on steel and aluminium and threatening taxes on billions more in Chinese goods.

They have said the administration should be working with other countries to pressure China, instead of wielding tariffs that invite retaliation on industries such as agriculture.

Senator Ben Sasse, a Republican who represents Nebraska and has been sharply critical of the tariffs, said it was "good news" that the president directed top staff, including US Trade Representative Robert Lighthizer, to negotiate US entry into TPP.

"The best thing the United States can do to push back against Chinese cheating now is to lead the other 11 Pacific nations that believe in free trade and the rule of law, " he said.

The TPP, a trade deal that was to involve 12 countries including the US, was supported by former President Barack Obama as a way to counter China's surging power in the region.

Labour unions and others had criticised it as too favourable to business. Mr Trump's Democratic rival in the 2016 election, Hillary Clinton, also came out against the agreement during the campaign.

Withdrawing from the deal was one of Mr Trump's first acts as president, delivering one of his core campaign promises.

After he withdrew, the remaining 11 countries continued to negotiate over the pact, signing the deal in March.

Exporters, such as farmers, have said they are now concerned that the US will be at a disadvantage to competitors in the region.

Mr Trump ordered his staff to evaluate rejoining "on our terms", according to accounts from the meeting. He has previously said he might reconsider the deal.