US President Donald Trump will approve the Iran nuclear deal only one more time before abandoning it if it is not changed, White House officials say.
The waiver he will sign suspends US sanctions on Iran for 120 more days.
A senior administration official said Mr Trump wants the 2015 Iran deal strengthened with a follow-on agreement before the 120 days are up, or the United States will unilaterally withdraw from the international pact.
The White House wants a deal with EU signatories to make restrictions on Iran's uranium enrichment permanent. Under the current deal they are set to expire in 2025.
Mr Trump also wants Iran's ballistic missile programme to be addressed.
"This is the last time he'll issue waivers unless they reach an agreement," senior White House officials said.
Mr Trump has strongly criticised the deal, which was reached during Barack Obama's presidency and helped end a long crisis. European powers say that the accord is vital for international security.
The US still maintains separate sanctions on Iran related to matters such as terrorism, human rights and ballistic missile development, and the Treasury Department announced it would impose new, targeted sanctions against 14 Iranian entities and individuals.
Mr Trump has argued behind the scenes that the nuclear deal makes the United States look weak, a senior US official said. The argument for staying in was to allow time to toughen the terms of the agreements, the official said.
Two senior Trump administration officials told Reuters that the president, a Republican, had privately expressed reluctance to heed the advice of top advisers recommending he not reimpose the suspended sanctions.
Britain, France and Germany called on Mr Trump earlier in the week to uphold the pact.
Meanwhile an Iranian politician said Iran "will not mourn" if the US pulls out of the nuclear deal.
First Vice-President Eshaq Jahangiri said in a post on his Instagram page that the US would "lose out" if they withdrew from the deal.
Iran says the missiles it has tested are not designed to carry nuclear warheads and insists its nuclear programme is entirely peaceful.