Russia on Saturday said Washington would be making a grave mistake by pulling out of the Iran nuclear deal, adding that Moscow would work hard to keep the landmark agreement alive.

Deputy Foreign Minister Sergei Ryabkov criticised remarks by US President Donald Trump, who on Friday said the US will not reimpose nuclear sanctions on Iran for the moment, but would withdraw later this year unless the terms of the deal are changed."We are gradually coming to the conclusion that an internal decision by the US to leave the (Joint Comprehensive Plan of Action) has already been made or is close to being made," Ryabkov said in an interview with Interfax news agency.

"This could be one of Washington's big foreign policy mistakes, a big miscalculation in American policy," he said.

Under the hard-won 2015 deal with Russia, the US, China, France, Britain, Germany and the EU, Iran agreed to curb its nuclear programme in exchange for lifting of a raft of international sanctions.

Trump on Friday gave an ultimatum to "either fix the deal's disastrous flaws, or the United States will withdraw."

America's allies see the accord as the best way to thwart Iran's nuclear ambitions and a victory for multilateral diplomacy. Tehran categorically denies it is seeking to develop atomic weapons.

But Trump argues that his predecessor Barack Obama gave away too much to Iran in sanctions relief, without forcing the Islamic republic to end its ballistic missile program and support for militant groups.

Ryabkov said Moscow must unite with Europe and China and undertake "intense work" to keep the existing plan intact and decried what he said was a US attempt to strongarm the situation.

"In what we heard yesterday, I don't see any invitation for Iran to enter dialogue," he said. "This defies the logic of the agreement."

"Russia will do everything in its power to save the agreement," he said.