The US has insisted it will leave the Paris climate accord, despite reports that it may be softening its stance.
Officials who met a White House representative on Saturday said afterwards the US would either stay in the 2015 accord or change its approach.
The White House said "there has been no change" in the US position "unless we can re-enter on terms that are more favourable to our country".
President Donald Trump said in June he wanted a new "fair" deal for the US.
He added it was important a new deal would not disadvantage US businesses, but opponents say withdrawing from the accord is an abdication of US leadership on a key global challenge.
The Paris agreement commits the US and 187 other countries to keeping rising global temperatures "well below" 2C above pre-industrial levels and "endeavour to limit" them even more, to 1.5C.
Only Syria and Nicaragua did not sign up to the deal.
WHAT DID TRUMP ORIGINALLY ANNOUNCE IN JUNE?
Speaking in the White House Rose Garden, he characterised the Paris agreement as a deal that aimed to hobble, disadvantage and impoverish the US.
He claimed the agreement would cost the US 6.5 million jobs and $3tn in lost GDP - while rival economies like China and India were treated more favourably.
"In order to fulfil my solemn duty to protect America and its citizens, the United States will withdraw from the Paris climate accord... but begin negotiations to re-enter either the Paris accord or a really entirely new transaction on terms that are fair to the United States," he said.
During his visit to France in July, however, Mr Trump hinted that the US could shift its position on the deal - but did not elaborate.