WASHINGTON — The United States on Tuesday joined Israel in alleging "possible undeclared nuclear activities" by Iran, further straining European-led attempts to salvage a multinational deal.

Iran denounced the accusations leveled on Monday by Israeli Prime Minister Benjamin Netanyahu, who said that the clerical regime operated a previously undisclosed site aimed at developing nuclear weapons but destroyed it after it was detected.

US Secretary of State Mike Pompeo, without directly referencing Netanyahu, urged Iran to comply with the UN nuclear watchdog, the International Atomic Energy Agency.

"The Iranian regime's lack of full cooperation with @iaeaorg raises questions about possible undeclared nuclear material or activities," Pompeo tweeted. "The world won't fall for it. We will deny the regime all paths to a nuclear weapon."

The new charges come in a fraught political climate, with French President Emmanuel Macron leading efforts to save a 2015 nuclear accord with Iran from which President Donald Trump withdrew the United States.

Macron proposed a summit between Trump and the Iranian leadership, a prospect that sparked interest from the mogul-turned-president but which is adamantly opposed by Netanyahu, who is facing elections next week and sees Iran as an existential threat.

The acting head of the IAEA, Cornel Feruta, called Monday for Iran to "respond promptly" to questions from the agency.

But despite Pompeo's charges of lack of cooperation, the IAEA chief said his exchanges with Iranian officials have been "very substantial" and that he was "pleased with the tone and the input we received."

Daryl Kimball, executive director of the Arms Control Association, a non-governmental organization in Washington, said that the United States and Israel should recognize that IAEA access in Iran came thanks to the 2015 nuclear accord.

"If any IAEA member-state including the United States or Israel has credible information, they should give it to the agency rather than make a public-relations show out of this," Kimball said.

"As with many allegations about particular sites, it's the agency that needs to investigate because it's the only authority that has the technical means and objectivity to come up with the right conclusions," he said.

Kimball believed there was still a window for Macron's efforts to succeed, especially as Trump relishes an unorthodox approach to diplomacy. But he warned that time may be running out, with Iran taking a series of steps to come out of strict compliance with the 2015 accord.

In the latest move, the IAEA confirmed that Iran was installing centrifuges at its Natanz facility that were more advanced than those allowed under the nuclear deal.

Iran wants to take small but symbolic steps to show its disappointment that it has not reaped benefits from the nuclear accord, under which it was promised sanctions relief in return for compliance.

Trump has imposed sweeping unilateral sanctions that include a ban on all oil sales from Iran. — AFP