US intelligence agencies have warned that Iran has substantially stepped up efforts to target American diplomats and officials in the Middle East and other countries, with some plots reaching advanced stages, according to an intelligence memorandum obtained by Axios.

The warning comes amid heightened regional tensions, with Washington reducing staffing at several diplomatic missions following intensified attacks by Yemen's Iran-allied Houthi forces against Saudi Arabia.

The memorandum, dated October 23 and addressed to US Secretary of State Marco Rubio by Director of National Intelligence Jay Clayton, points to an increasingly dangerous security environment as the nearly eight-month-old regional conflict threatens to enter another phase.

"Specifically, through various channels, we have received intelligence that Iran has substantially increased efforts to target US personnel," Clayton wrote, according to the memorandum first reported by Axios journalist Barak Ravid.

"The lethal plotting efforts by the Iranians against US citizens have reached advanced stages in both the Middle East and certain other countries," he added.

CIA Director John Ratcliffe was also copied on the unclassified memorandum.

US orders diplomatic withdrawals

The State Department on Wednesday ordered non-emergency US government employees and their family members to leave American diplomatic missions in Saudi Arabia, Reuters reported, citing US officials.

The measures followed an escalation in Houthi attacks against Saudi Arabia, including strikes targeting strategic and energy-related facilities, raising concerns about the safety of foreign personnel and the potential for further disruption to regional economic activity.

US officials are increasingly concerned that the Houthis could widen their operations beyond Saudi Arabia to target other countries in the Gulf, opening additional fronts in an already volatile conflict.

The latest developments have coincided with renewed attacks on commercial shipping in the Strait of Hormuz, a critical maritime passage for global energy supplies.

According to Axios, the one-page memorandum also raised concerns that intelligence about the Iranian plots was not reaching enough American personnel who could potentially be at risk.

Clayton said the underlying intelligence had already been shared with senior State Department officials and offered to lower the classification of as much of the sensitive material as possible to facilitate wider circulation.

The document did not identify specific targets, countries outside the Middle East or the precise nature of the alleged plots.

A US official subsequently confirmed the authenticity of the memorandum

The State Department has meanwhile expanded its precautionary measures beyond Saudi Arabia.

According to Axios, citing a US official, Washington on Friday ordered reductions in diplomatic and government personnel at its embassies in the UAE, Lebanon, Qatar and Jordan.