Moscow warned on Monday there would be "negative consequences" if US President Donald Trump fails to uphold the landmark Iran nuclear deal negotiated by his predecessor.

Trump is a fierce critic of the 2015 accord, which he has called "the worst deal ever", and US officials say he intends to tell Congress next week that Tehran is not honouring its side of the bargain.

"Obviously if one country leaves the deal, especially such a key country as the US, then that will have negative consequences," Russian President Vladimir Putin's spokesman said.  

"We can only try to predict the nature of these consequences, which we are doing now," Dmitry Peskov told journalists.

Putin has repeatedly hailed the importance of the existing deal, he added.

Trump is expected to announce that he is "decertifying" Iran's compliance with the agreement it signed to limit its nuclear programme in exchange for sanctions relief.

US officials insist this will not sink the deal itself but open the way for Congress to possibly develop new measures to punish other aspects of Iran's behaviour.

Resumed sanctions could derail the accord negotiated with Tehran by former president Barack Obama and other major world powers.

Congress requires the president to certify Iranian compliance with the deal every 90 days. The next certification date is October 15.

Under the law, Congress would then have 60 days to decide whether to reimpose sanctions lifted by the deal.