US President Donald Trump is confident he could still renegotiate the 2015 nuclear accord with Iran, despite the killing of Tehran's top general in a US air raid last week, White House adviser Kellyanne Conway said on Monday.

Conway made the comment to reporters at the White House on Monday, a day after Iran announced it would retreat further from the 2015 nuclear pact following the killing of its top general in a US air raid at the Baghdad international airport last week.

Iranian officials said they would no longer abide by enrichment limits under the agreement, representing the clearest nuclear proliferation threat yet made by Tehran since Trump unilaterally withdrew from the accord in May 2018 and implemented a policy of "maximum pressure" sanctions against Iran. The announcement was decried by European officials trying to salvage to the flagging deal.

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Asked if Trump believes he can still get Iran to negotiate a new nuclear agreement, Conway told reporters at the White House: "He said he's open. If Iran wants to start behaving like a normal country ... sure, absolutely."

Trump later took to Twitter to reiterate the White House stance that "Iran will never have a nuclear weapon" but gave no other details.

Tensions between Tehran and Washington escalated after Trump unilaterally withdrew the US from the 2015 nuclear agreement, which was reached between Iran, the five permanent members of the United Nations Security Council and Germany. Under the agreement, Iran agreed to curb its nuclear programme in return for sanctions relief.

Iran has repeatedly said it will not renegotiate the nuclear deal.

Tehran has incrementally moved away from its commitments under the agreement, saying the European parties have failed to find a workaround to the US sanctions. However, while announcing they would no longer abide by the enrichment limits, Iran said it would still continue to cooperate with the United Nations nuclear watchdog and could quickly reverse its steps if US sanctions are removed.

On Monday, Conway also defended Trump's decision to kill top Iranian General Qassem Soleimani, an act that has threatened to bring long-simmering tensions between the two countries to a head.

The president "did what a responsible, strong - not weak - commander-in-chief does when faced with the opportunity to take out one of the - if not the - world's most wanted terrorists," Conway said.