As U.S. and Iranian diplomats inched toward progress on Tehran’s nuclear program last week, Saudi Arabia quietly signed its own nuclear-cooperation agreement with South Korea.

That agreement, along with recent comments from Saudi officials and royals, is raising concerns on Capitol Hill and among U.S. allies that a deal with Iran, rather than stanching the spread of nuclear technologies, risks fueling it.

Saudi Arabia’s former intelligence chief, Prince Turki al-Faisal, a member of the royal family, has publicly warned in recent months that Riyadh will seek to match the nuclear capabilities Iran is allowed to maintain as part of any final agreement reached with world powers. This could include the ability to enrich uranium and to harvest the weapons-grade plutonium discharged in a nuclear reactor’s spent fuel. […]