The UN's nuclear watchdog said Wednesday it had gained access to a second site in Iran where undeclared nuclear activity may have take place in the early 2000s.

"As part of an agreement with Iran to resolve safeguards implementation issues specified by the IAEA, the agency this week conducted a complementary access at the second location in the country and took environmental samples," the International Atomic Energy Agency said in a statement.

Late last month Iran announced it would allow the IAEA access to two sites -- their exact locations have not been made public -- following a visit to Tehran by IAEA Director General Rafael Grossi.

The first site was visited by inspectors in early September.

Iran had denied the agency access to the locations earlier this year, prompting the IAEA's board of governors to pass a resolution in June urging Iran to comply with its requests.

The row had threatened to put yet more strain on the 2015 landmark Iran nuclear deal, which has hung by a thread since the US pulled out of it in 2018.

At a press conference on September 14, Grossi said analysing environmental samples collected at the first site would take "not less than a couple of months, two or three months maybe".