Oct 24 (Reuters) - Iran has ramped up efforts to target US personnel, and lethal plots by Tehran against American citizens have reached "advanced ​stages", a senior intelligence official in the Trump administration warned in a memo seen ‌by Reuters.

The alert from Director of National Intelligence Jay Clayton came after Washington scaled back staffing at its mission in Saudi Arabia earlier this week in response to intensified attacks by Iran-allied Houthi forces.

Those attacks on one ​front of the nearly eight-month-old regional conflict triggered by the Iran warhave coincided with a rise in ​attacks on commercial shipping in the Strait of Hormuz.

Earlier this month, sources told ⁠Reuters Iran was preparing a more forceful response if the United States resumes large-scale military attacks on ​its territory.

"Specifically, through various channels, we have received intelligence that Iran has substantially increased efforts to ​target US personnel," Clayton wrote to US Secretary of State Marco Rubio in a message dated October 23.