Satellite imagery recently emerged of the project on the outskirts of Riyadh, which comes amid controversy in Washington over what Democrats say is President Donald Trump's rush to approve nuclear projects with the oil-rich kingdom.

But Yukiya Amano, director general of the International Atomic Energy Agency, said there was nothing secret about the reactor and that Saudi Arabia informed the Vienna-based UN body about its plans in 2014.

He said the IAEA has encouraged Saudi Arabia to sign a comprehensive safeguards agreement, under which the agency ensures that nuclear material is not being diverted to weapons use.

Saudi Arabia has a weaker accord designed for countries with minimal quantities of nuclear material -- which Amano said was fine until the kingdom imports significant amounts.

"We have proposed to Saudi Arabia to rescind and replace it by the full-fledged comprehensive safeguards agreement," Amano told reporters in Washington.

"They didn't say no, they didn't say yes, and they are now giving thoughts. We are waiting," he said.

"For now, they don't have the material, so there is no violation," he said.

Amano said that Saudi Arabia may bring in nuclear material "by the end of the year," although he cautioned that nuclear projects frequently get delayed.

US Energy Secretary Rick Perry told a Senate hearing last month that his department had given the go-ahead for six applications by US companies to do nuclear work in Saudi Arabia.

The approvals come even though Saudi Arabia has not sought a so-called Section 123 Agreement to guarantee the peaceful use of nuclear technology, which is required under US law before any transfer of sensitive material.

Saudi Arabia has come under sharp criticism in the United States over the civilian death toll in its offensive in Yemen and for the murder and dismemberment of US-based dissident writer Jamal Khashoggi.

"If you cannot trust a regime with a bone-saw, you should not trust them with nuclear weapons," Democratic Representative Brad Sherman told Secretary of State Mike Pompeo at a hearing last week.

Saudi Arabia's powerful crown prince, Mohammed bin Salman, has warned that the kingdom would seek nuclear weapons if its arch-rival Iran obtains one.