Flags of U.S. and Russia
Flags of U.S. and Russia iStock

Russia warned on Monday there would be "negative consequences" if U.S. President Donald Trump fails to uphold the Iran nuclear deal.

Trump has been a fierce critic of the 2015 accord, which was negotiated under his predecessor. Barack Obama.

He has called the agreement "the worst deal ever", and U.S. officials say he intends to tell Congress next week that Tehran is not honoring its side of the bargain, paving the way for new sanctions on the Islamic Republic.

"Obviously if one country leaves the deal, especially such a key country as the U.S., then that will have negative consequences," Russian President Vladimir Putin's spokesman, Dmitry Peskov, said Monday, according to AFP.

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

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

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

Democrats, and even some senior Trump administration officials, backed recertification and the continuation of the deal.

Some 180 congressional Democrats signed a letter to the president urging him to recertify Iran's compliance with the nuclear deal ahead of the October 15 deadline.

Similarly, U.S. Defense Secretary James Mattis opined last week that it would be in America's national security interest to stay in the Iran nuclear deal.

Iranian officials have threatened to respond if Trump declares that Iran is not complying with the deal.