Moscow has offered to mediate between the United States and Iran after U.S. President Donald Trump threatened to bomb Iran unless it comes to an agreement with Washington over its nuclear programme.

"We believe that the problem of the Iranian nuclear dossier should be discussed and resolved only by political and diplomatic means. Exclusively," Kremlin spokesman Dmitry Peskov told reporters.

"And, of course, we believe that all parties should maintain absolute restraint here and focus specifically on diplomatic efforts when discussing all issues.

"You know that we are currently working on restoring our relations with the United States, but Iran is also our partner, our ally, with whom we have very developed and multifaceted relations," Peskov said.

His comments underscored the balance that Moscow is attempting to strike as tensions rise sharply over Iran's nuclear activities.

Russia has deepened ties with Tehran since the start of the war in Ukraine and signed a strategic partnership agreement with the Islamic Republic in January.

But it is also moving quickly to restore relations with the Trump administration, a shift that has unnerved Ukraine and European countries who are nervous that Trump could make deep concessions to Moscow in order to end the conflict in Ukraine.

Trump says Iran is too close to building a nuclear bomb. Tehran denies any such intention and says it wants to develop nuclear energy for peaceful purposes, something that Russia says it has the right to do.