(Reuters) -- Russia urged the United States not to interfere in what Moscow calls Iran's "domestic affairs", Deputy Foreign Minister Sergei Ryabkov said on Thursday, according to the TASS news agency.

Anti-government demonstrations erupted a week ago in Iran, and U.S. President Donald Trump has expressed support for the protesters. At least 21 people have died in the demonstrations, which began as protests against economic hardship.

Iran's army chief said on Thursday that police forces had quelled the unrest, but his troops were ready to intervene if needed.

"I am certain that our neighbour, our friendly state will manage to overcome current difficulties and come from the current period as a strengthened country and reliable partner to solve various problems", Ryabkov said, according to TASS.

Those problems included compliance with the agreement Iran reached with world powers in 2015 to curtail its nuclear-power programme. Trump has long criticised that agreement.

"The current situation, when Washington gives in to the temptation to raise more questions" on the nuclear agreement, Ryabkov said, is evidence that the United States is trying to undermine adherence to the deal.

He also said Moscow is sticking to the position that the deal "is not to be corrected", playing down Washington's stance on the agreement.

In October, Trump declined to certify that Iran was complying with the nuclear deal. He must decide in mid-January if he wants to continue to waive energy sanctions on Iran .