Following U.S. President Donald Trump withdrawing from a major nuclear pact and on Tuesday resuming Iran sanctions, a spokesman for UN Secretary-General Antonio Guterres said bilateral relations are up to the governments involved.

"The secretary-general has, himself, repeatedly discussed the importance of the Joint Comprehensive Plan of Action (JCPOA) as one of the key diplomatic achievements of recent years. He believes that it deserves continued support and that all of the parties to that agreement need to abide by its terms," spokesman Farhan Haq said.

"Regarding how the nations, the governments, deal with this issue, obviously, we don't speak to the national domestic policy of each country," the spokesman said.

Trump withdrew from the nuclear deal with Tehran and European countries and on Tuesday his orders to reinstate sanctions, suspended under the JCPOA, took effect.

Washington received widespread criticism for withdrawing from the European-Iran-U.S. accord which limited Tehran's nuclear capability.

Trump's action fulfilled a campaign promise to tear up the agreement which he took as one of the worst ever negotiated.

"There was an agreement (the JCPOA) put into place and the secretary-general's views on that agreement are well known," Haq said. "We will continue as far as we can to support implementation of the JCPOA which as you know also involves the activity of the International Atomic Energy Agency (IAEA)."