The Trump administration on Friday cited a national security "emergency" allegedly caused by Iran to bypass Congress and rush through arms sales worth billions of dollars to Saudi Arabia and other Middle East allies, in a move that drew condemnation from lawmakers on both sides of the aisle.

Citing a rarely used provision of arms control law, the administration informed lawmakers it was declaring a national security emergency, allowing it to go ahead with the sale of weapons to Saudi Arabia, the United Arab Emirates and Jordan without congressional approval, according to administration letters sent to senators and obtained by NBC News.

"I have determined that an emergency exists which requires the proposed sale in the national security interest of the United States, and, thus, waives the congressional review requirements," Secretary of State Mike Pompeo wrote in a letter to Sen. James Risch, the Republican chairman of the Senate Foreign Relations Committee.

The decision affected various arms packages worth roughly $8 billion, including deals for precision-guided bombs and related gear for Saudi Arabia and the UAE, according to the documents and congressional aides.

The two countries are staunch U.S. allies that support President Donald Trump's policies on Iran and have been waging a war since 2015 in support of the Yemeni government against Houthi rebels backed by Tehran.

The move came despite growing bipartisan opposition to any arms sales to Saudi Arabia amid outrage over the killing of Saudi writer Jamal Khashoggi at the Saudi consulate in Istanbul last year, as well as over Riyadh's air war in Yemen that has caused high numbers of civilian casualties.

A bipartisan majority in Congress has voted to halt U.S. support for the Saudi-led war in Yemen but President Donald Trump vetoed the legislation last month.

A memo accompanied Pompeo's letters justifying the declaration of the emergency due to Iran's actions, including its support for Houthi rebels in Yemen fighting the Saudi-led coalition.

"Iranian malign activity poses a fundamental threat to the stability of the Middle East and to American security at home and abroad," the memo states. "Current threat reporting indicates Iran engages in preparations for further malign activities throughout the Middle East region, including potential targeting of U.S. and allied military forces in the region."

Iran has accused the U.S. of trying to provoke a war and denied any role in recent attacks on ships near the coast of the UAE or on a pipeline in Saudi Arabia.

Pompeo said in a statement that delaying the arms shipments, which included bombs, parts for fighter jets and other hardware, could cause problems for allied aircraft and call into question U.S. reliability in providing equipment.

"The United States is, and must remain, a reliable security partner to our allies and partners around the world," Pompeo said.

But the secretary of state said the decision to bypass Congress was a “a one-time event” and that the administration would uphold the long-established process for congressional review of proposed arms sales.

Democrats in Congress said the Trump administration expedited the arms packages because it could not secure a majority of lawmakers to support any proposed sales to the Saudis.

"President Trump is only using this loophole because he knows Congress would disapprove of this sale. There is no new 'emergency' reason to sell bombs to the Saudis to drop in Yemen, and doing so only perpetuates the humanitarian crisis there," Democratic Sen. Chris Murphy of Connecticut said in a statement.

Some Republicans also denounced the White House for circumventing Congress to complete the sale.

“I understand the administration’s frustration that key members of Congress held these arms sales for an extended period of time, in some cases for over a year," said Rep. Mike McCaul of Texas, the top Republican on the House of Representatives Foreign Affairs Committee.

"However, the President’s decision to use an emergency waiver on these sales is unfortunate and will damage certain future congressional interactions."

Republican Sen. Todd Young of Indiana called on the administration to reconsider the decision.

"I strongly urge the administration to reverse course from bypassing congressional oversight on arms sales to Saudi Arabia," Young said.

"Iran remains the world's largest state sponsor of terror but the current threats that have been briefed to members of Congress do not justify taking this dramatic step. "

Sen. Bob Menendez, the ranking Democrat on the Senate Foreign Relations Committee, questioned whether the decision was legal and accused the Trump administration of flouting congressional authority while granting favors to Gulf governments accused of human rights abuses and alleged indiscriminate bombing in Yemen.

"I am disappointed, but not surprised, that the Trump Administration has failed once again to prioritize our long term national security interests or stand up for human rights, and instead is granting favors to authoritarian countries like Saudi Arabia," Menendez said in a statement.

He said "the Trump administration decided to do an end run around the Congress and possibly the law."

Menendez had held up the sale of tens of thousands of precision-guided bombs to Saudi Arabia and the UAE for a year, due to concerns about civilian deaths from Saudi-led airstrikes, the killing of Khashoggi and alleged rights abuses linked to the UAE in the war in Yemen.

Rights advocates and humanitarian groups also condemned the decision.

"The Trump Administration is manufacturing an emergency to push through the sale of deadly weapons to Saudi Arabia and the United Arab Emirates," Scott Paul of Oxfam America said. "The real emergency is the 12 million people at risk of famine in Yemen and the largest-ever recorded cholera outbreak continues to spread because of the conflict, but this administration shows little concern for the millions who suffer."

Apart from precision-guided munitions or so-called "smart bombs," the arms sales for Saudi Arabia include mortar bombs, engines and maintenance support for F-15 fighter jets and logistical services for the Saudi air force, according to documents sent to Congress from the administration.

The arms packages for the United Arab Emirates cover precision-guided bombs, equipment for AH-64 helicopters, laser-guided rockets, javelin anti-tank missiles, .50 caliber semi-automatic rifles, Patriot missiles, F-16 fighter jet engine parts and U.S. Marine Corps training of the country’s presidential guard. The weapons sale for Jordan involved a transfer of Paveway precision-guided bombs from the Emirates.

Saudi Arabia's embassy in Washington did not respond to a request for comment.

The Trump administration has refrained from public criticism of the Saudi-led coalition's campaign in Yemen and has focused on Iran's support of Houthi rebels in the conflict, accusing Tehran of fueling the war.

But some experts and former officials say the war in Yemen benefits Iran and Al Qaeda-linked militants and that the U.S. needs to use its influence with the Saudis to bring an end to the fighting.

"The longer the civil war in Yemen continues, the more opportunity Tehran will have to undermine the interests of the U.S. and our security partners," said Bradley Bowman of the Foundation for the Defense of Democracies, a Washington think tank.

"It is in U.S. national security interests to end the civil war in Yemen and address the horrible humanitarian crisis there — both of which are pushing the Houthis deeper into the welcoming arms of Tehran."

Menendez and other lawmakers said they would look at a possible legislative response to the Trump administration's decision. Two Democratic congressional aides said senators were discussing legislation that would possibly bar future arms sales to Saudi Arabia without congressional approval.

The White House move could trigger a backlash in Congress that would jeopardize future arms sales, Bowman said.

"An administration end-run around Congress to complete arms sales to Riyadh risks inciting a congressional reaction that will undermine the administration's broader goals related to its conventional arms transfer policy," he said.

Under U.S. arms control law, Congress must be given 30 days to approve U.S. arms sales to foreign countries. However, in a rarely used provision of that law, the president can declare an "emergency," sidestepping Congress and sending the sale through immediately.

In 1984, President Ronald Reagan used the same provision to sell 400 Stinger missiles and 200 launchers to Saudi Arabia in response to its urgent request for help in defending the kingdom against Iran.

Saudi Arabia remains the United States' largest foreign military sales customer with more than $129 billion in approved purchases.