Satellite images released Thursday show "unusual activity" around Iran's Fordo nuclear enrichment facility in Qom, southwest of Tehran.
The facility, built under a mountain, was supposed to be converted into a research center as part of the 2015 nuclear deal.
The images, taken on April 29 by Israeli firm ImageSat International, show a large number of vehicles at the entrance to the facility and other signs of increased activity there, as well as two buildings that did not exist in 2016, when the area was last photographed.
It is believed the buildings are new research and development facilities.
The images do not in themselves indicate that Iran is in violation of its agreement with world powers, formally known as the Joint Comprehensive Plan of Action.
U.S. President Donald Trump said he would decide whether to renew waivers suspending some of the U.S. sanctions on Iran by May 12. One White House official said it was possible Trump will end up with a decision that "is not a full pullout" but was unable to clarify that.
Iranian Foreign Minister Mohammad Javad Zarif on Thursday rejected any form of renegotiation.
"Iran will not renegotiate what was agreed years ago and has been implemented," he said in a video message posted on YouTube.
"Let me make it absolutely clear and once and for all: We will neither outsource our security nor will we renegotiate or add onto a deal we have already implemented in good faith."
A senior adviser to Iranian Supreme Leader Ayatollah Ali Khamenei also warned the Europeans on Thursday against "revising" the nuclear deal.
"Even if U.S. allies, especially the Europeans, try to revise the deal ... one of our options will be withdrawing from it," state television quoted Ali Akbar Velayati as saying.
