Donald Trump is bypassing Congress to clear the sale of $7 billion-worth of arms to Saudi Arabia citing the growing threat to the region from Iran.
The Trump administration has invoked a rarely used provision in US federal law to bypass congressional review of arms sales.
Secretary of State Mike Pompeo notified Congress of the decision to use an emergency loophole in the Arms Export Control Act to move ahead with sales of $7 billion (£5.4 billion) in weapons and aircraft maintenance support to Saudi Arabia, along with the United Arab Emirates and Jordan, without legislators' approval.
In his notification, Mr Pompeo said he had made the determination "that an emergency exists which requires the immediate sale" of the weapons "in order to deter further the malign influence of the government of Iran throughout the Middle East region".
He said the transfers "must occur as quickly as possible in order to deter further Iranian adventurism in the Gulf and throughout the Middle East".
Mr Trump is due to land in Japan on Saturday for a state visit and is expected in the UK at the beginning of June.
Mr Pompeo's move follows President Trump's announcement that the US plans to send 1,500 additional US troops to the Middle East as part of a build-up in response to an unspecified threat from Iran.
It also comes as the administration has actively courted close ties with Saudi Arabia over congressional objections, notably following the killing of Jamal Khashoggi , a US-based columnist for The Washington Post, by Saudi agents in October.
Mr Khashoggi's killing, coupled with increasing concerns about civilian casualties resulting from a Saudi-led coalition's military operation against Iran-backed Houthi rebels in Yemen, prompted legislators to block about 2 billion dollars (£1.6 billion) in arms sales to the kingdom for more than a year.
Last month, Mr Trump vetoed legislation that would have ended US military assistance for the Saudi-led war in Yemen.
Critics of the Saudi campaign quickly denounced Friday's step.
Senator Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, said the administration did not cite a specific legal or practical reason for using the loophole other than Iran.
"I am disappointed, but not surprised, that the Trump administration has failed once again to prioritise our long-term national security interests or stand up for human rights, and instead is granting favours to authoritarian countries like Saudi Arabia," Mr Menendez said in a statement.
Senator Chris Murphy, who earlier this week warned against bypassing Congress, said the administration was only declaring an emergency because legislators would have blocked the transfers.
"President Trump is only using this loophole because he knows Congress would disapprove of this sale," Mr Murphy said. "There is no new 'emergency' reason to sell bombs to the Saudis."
The law requires Congress to be notified of potential arms sales, giving the body the opportunity to block the sale.
But the law also allows the president to waive that review process by declaring an emergency that requires the sale be made "in the national security interests of the United States."