Democratic Senator Ed Markey, of Massachusetts, says any deal is "almost certain" to require a non-proliferation accord, known as a "123 agreement," of the type the United States has previously signed with South Korea and India, and which is designed to prevent the spread of nuclear weapons.

"Previous US efforts to conclude a 123 agreement with Saudi Arabia have been unsuccessful because of its long-standing refusal to commit to foregoing any uranium enrichment or spent-fuel reprocessing on its territory -- the so-called... 'gold standard' for 123 agreements," Markey, of the Senate Foreign Relations Committee, said in a letter to Secretary of State Rex Tillerson and Energy Secretary Rick Perry.

AFP on Tuesday obtained a copy of the letter, which is dated February 26.

Riyadh plans to announce at the beginning of March its short list of firms which will bid to build its nuclear reactors.

Besides the US company Westinghouse, Russian, French, Chinese and South Korean firms are in the running.

A nuclear accord between Riyadh and Washington would allow US corporations to export their nuclear technology to Saudi Arabia, while tensions are high surrounding the civil nuclear program of Riyadh's regional rival Iran.

US President Donald Trump has threatened to tear up a 2015 global pact under which Iran -- facing suspicions it was working towards a nuclear bomb -- agreed to curb its nuclear program in exchange for a lifting of sanctions.

Both Washington and Riyadh have complained of Iran's "destabilizing" acts in the Middle East.

Markey says Saudi Arabia's "unwillingness" to commit to a "gold standard" 123 agreement "is particularly concerning in light of comments made by Saudi officials and members of the royal family suggesting that a nuclear program may be as much for geopolitical purposes as for electricity generation."

According to several US media reports, Saudi Arabia's Crown Prince Mohammed bin Salman -- the main driver of a more aggressive regional push by the kingdom -- is to visit the United States in early March to meet with Trump.

The visit has not been officially confirmed by either country.

Ties between the kingdom and Washington have strengthened since Trump assumed office early last year. His first official trip abroad was to Saudi Arabia, which is trying to diversify its oil-based economy and energy sources.