After President Donald Trump on Thursday ominously warned that Iran made a "very big mistake" by downing a U.S. drone and told reporters they will "soon find out" whether the administration plans to take military action, advocacy groups warned that any strike against Iran "could spark a major war" and urged Congress to act to avert a potentially catastrophic conflict.

"Congress needs to bar the door to war and make clear that there is no authorization for any military strikes against Iran."

—Jon Rainwater, Peace Action

"Congress needs to grill the administration about how retaliatory strikes could spiral into lethal war," Jon Rainwater, executive director of Peace Action, said in a statement. "As importantly, Congress needs to bar the door to war and make clear that there is no authorization for any military strikes against Iran."

"Donald Trump and [national security adviser] John Bolton can't be trusted in such a dangerous situation and they may need to be pushed kicking and screaming towards deescalation," Rainwater added. "We need to return to diplomacy to deescalate the situation and address the substantive issues behind this conflict. Many of us predicted that walking away from the Iran deal would lead us to the brink of war. The window for averting war is closing but it's not too late to step back and pursue a more sober path."

Jamal Abdi, president of the National Iranian American Council, echoed Rainwater, saying in a statement that "Congress needs to step in and make clear that Trump does not have authorization to start a new war."

"There is still time for Trump to defuse tensions with Iran and put to rest this manufactured crisis," Abdi said. "Rather than opt for the military options that Bolton will undoubtedly propose, Trump should seek out third party mediators who can help deescalate and bring the U.S. and Iran back to the negotiating table."

The anti-war groups' warnings came after Trump told reporters in front of the White House that they will learn shortly whether he plans to launch a military strike against Iran after it downed a U.S. surveillance drone. Iran said the drone violated its airspace.

JUST IN: Asked whether he would strike Iran in retaliation for downing a US drone, Pres. Trump says: “You’ll soon find out.” https://t.co/7VO0Jne8eT pic.twitter.com/HH6ZQhT0qP — ABC News (@ABC) June 20, 2019

Earlier Thursday morning, vaguely warned on Twitter that "Iran made a very big mistake":

Iran made a very big mistake! — Donald J. Trump (@realDonaldTrump) June 20, 2019

Critics were quick to raise alarm about Trump's tweet and point out that the president's hawkish actions towards Iran—including his violation of the nuclear deal last year—are to blame for escalating military tensions.

"Pulling out of the Iran nuclear deal was a very big mistake," tweeted journalist and New York Times contributor Wajahat Ali. "Escalating tensions when there were diplomatic relations was a very big mistake. War with Iran will be a very big mistake."

Pulling out of the Iran Nuclear Deal was a very big mistake. Escalating tensions when there were diplomatic relations was a very big mistake. War with Iran will be a very big mistake. https://t.co/X8p2BLaM4b — Wajahat Ali (@WajahatAli) June 20, 2019

Shortly after U.S. military officials confirmed that Iran shot down an American drone Thursday morning, Navy Captain and CENTCOM spokesman Bill Urban said in a statement that the downing of the aircraft was an "unprovoked attack on a U.S. surveillance asset in international airspace."

Urban did not provide evidence to refute Iran's claim that the U.S. aircraft violated Iranian airspace.

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In a tweet on Thursday, Iranian Foreign Minister Javad Zarif said Iran will "take this new aggression to U.N. and show that the U.S. is lying about international waters."

The US wages #EconomicTerrorism on Iran, has conducted covert action against us & now encroaches on our territory. We don't seek war, but will zealously defend our skies, land & waters. We'll take this new aggression to #UN & show that the US is lying about international waters — Javad Zarif (@JZarif) June 20, 2019

Sina Toossi, Research Associate at NIAC, warned in a statement that the drone incident represents "yet another potential trip mine to all-out war."

"If it is proven that Iran shot down the U.S. drone over international waters, it is a provocative act that must be condemned by the international community," Toossi said. "Regardless, there is a vital need for immediate U.S.-Iran deescalation. There are no military solutions to the U.S. disputes with Iran—only diplomatic ones."

"However," Toossi added, "rather than pursue sincere diplomacy, President Trump has elected to pile on pressure with no strategic foresight at the behest of uber-hawkish advisors like John Bolton."

During a press conference on following Trump's tweet Thursday, House Speaker Nancy Pelosi (D-Calif.) said members of Congress from both parties will receive a briefing on the drone incident and warned against "reckless" action.

"I think it's a dangerous situation," Pelosi said, adding that she doesn't think "the president wants to go to war."

"There is no appetite for going to war in our country," said Pelosi.

NEW: House Speaker Nancy Pelosi responds to escalating tensions with Iran following a US drone being shot down in the region: “I think it’s a dangerous situation... We have to be strong and strategic about how we protect our interests.” https://t.co/WEGrzTaZAY pic.twitter.com/NnLgJcBWqe — ABC News (@ABC) June 20, 2019

The downing of the U.S. drone comes as the Trump administration is reportedly laying the groundwork for a military attack on Iran without congressional approval.

As Politico reported on Wednesday, Sen. Tom Cotton (R-Ark.)—who over the weekend called for a "military strike" against Iran—"has spoken to several high-ranking U.S. officials and the president himself about the rising tensions with Iran after the U.S. blamed the Islamic Republic for attacking two oil tankers."

NIAC's Toossi said that if Trump truly wants to avoid war with Iran—as he has claimed several times over the past month—"he needs new advisers that would reopen channels of dialogue and enact policies that would bring Trump closer to a deal with Tehran, not war."