Speaking in Beijing on Thursday, Chinese Foreign Ministry spokesman Wang Wenbin said China opposed the U.S. proposal.
If the United States is unsuccessful in extending the embargo, it has threatened to trigger a return of all U.N. sanctions on Iran under a process agreed in the 2015 deal.
Such a move would kill the deal, touted as a way to suspend Tehran's suspected drive to develop nuclear weapons. Washington argues it can trigger the sanctions because a Security Council resolution still names it as a participant.
Iran has breached parts of the nuclear deal in response to the U.S. withdrawal and Washington's reimposition of sanctions.
"For as long as Iran is allowed to enrich, we're going to be having this discussion - how close is Iran to a nuclear breakout? ... We need to restore the U.N. Security Council standard of no enrichment," U.S. Iran envoy Brian Hook told the Aspen Security Forum, held virtually, earlier on Wednesday.
Iran denies it is seeking to build a nuclear bomb.
Diplomats say Washington would face a tough, messy battle if it tries to trigger a return to sanctions.
The United States would have to submit a complaint to the council, which would then have to vote within 30 days on a resolution to continue Iran's sanctions relief. If such a resolution is not put forward by the deadline, sanctions would be reimposed - what is known as a snapback.Some diplomats have suggested the United States will submit its complaint by the end of August to ensure the 30 days ends in September, before Russia takes the monthly rotating council presidency in October.