The State Department described its communication as a “strong message” to the world, following Donald Trump’s decision in June to leave the accord.
However, the move was played down by Nigel Purvis, who directed climate diplomacy during the Clinton and George W. Bush administrations.
“The State Department is telling the UN what the President already told the world on June 1 and it has no legal effect,” he said.
Mr Purvis said countries can’t withdraw from new international pacts, including the Paris climate one, until three years after they go into effect.
The Paris agreement went into effect on 4 November 2016.
The State Department cited the same timeline, saying it can officially start withdrawing as soon as November 2019.
In a statement, the department said it will continue to participate in international negotiations on current and future climate change deals.
The next meeting is in Bonn, Germany, in November.
Mr Trump is “open to re-engaging in the Paris Agreement if the US can identify terms that are more favourable to it, its business, its workers, its people and its taxpayers,” the department said.
Under the agreement, countries set their own national plans for cutting climate emissions.
That means Mr Trump can come up with different targets for the US than those set by President Barack Obama.