U.S. Secretary of State Mike Pompeo on Sunday defended President Donald Trump's decision to kill Iranian military commander Qassem Soleimani, as Democratic lawmakers questioned whether the action was justified and the United States braced for retaliation.

Democratic critics in Congress said it was not clear to them why the Iranian military leader, long seen as a threat by U.S. authorities, had to be killed now. They said Trump does not have the authority to go to war without congressional approval, and that his actions put the country at greater risk.

Trump's fellow Republicans in Congress have generally backed his decision.

Pompeo, Trump's top diplomat and adviser, made the administration's case during multiple television interviews on Sunday, saying there was "no scepticism" among senior U.S. leaders who had access to all the intelligence on Friday's targeted killing.

He deflected questions about the "imminent attack" he had cited on Friday as justification for the strike, and described the threat posed by Soleimani as long-term and wide-ranging.

"We would have been culpably negligent had we not taken this action," Pompeo said on NBC's Meet the Press.

WATCH: Pompeo defends U.S. air strike that killed Soleimani

U.S. Secretary of State Mike Pompeo defended on Sunday the U.S. air strike which killed Iranian military commander Qassem Soleimani, warning that the U.S. will respond if Iran retaliates. 1:21

"It's never one thing. ... It's never one moment. It's never one instance," he said. "It's a full situational awareness of risk and analysis."

U.S. Representative Adam Schiff, the Democratic chairman of the House Intelligence Committee, said Soleimani had been plotting against the United States for decades and that lawmakers had not received enough information on the alleged threat to convince him the killing was warranted.

"So the question is, why now?" he asked on CNN's State of the Union. "I don't think the intelligence supports the conclusion that killing a top Iranian official is going to either stop [the] plotting or improve American security."

Pompeo repeatedly said Soleimani's killing made Americans safer, but he also tallied the steps the U.S. is taking to guard against Iranian retaliation, including warning Americans to leave the region, boosting the U.S. military presence and shoring up cybersecurity systems.

On Sunday evening, Trump continued to threaten "major retaliation" against Iran if Tehran were to retaliate.

The president also told reporters travelling with him from Florida that he was willing to go after Iranian cultural sites because Iran had killed Americans, and he said the administration "may discuss" releasing intelligence related to the killing of Soleimani.

Representative Adam Schiff said U.S. lawmakers had not received enough information on the alleged threat to convince him the assassination of Soleimani was warranted. (Saul Loeb/AFP/Getty Images)

Trump also threatened sanctions against Baghdad after Iraq's parliament called on U.S. troops to leave the country, and the president said if troops did leave, Baghdad would have to pay Washington for the cost of an air base there.

"We have a very extraordinarily expensive air base that's there. It cost billions of dollars to build, long before my time. We're not leaving unless they pay us back for it," Trump told reporters on Air Force One.

Trump said that if Iraq asked U.S. forces to leave and it was not done on a friendly basis, "we will charge them sanctions like they've never seen before ever. It'll make Iranian sanctions look somewhat tame."

Long-running hostilities

The attack that killed Soleimani at the Baghdad airport took long-running hostilities between Washington and Tehran into uncharted territory and raised the spectre of wider conflict in the Middle East.

Senator Elizabeth Warren, a candidate for the Democratic nomination to face Trump in November's presidential election, noted the strike came as Trump faced a Senate trial after his impeachment by the Democratic-led House of Representatives. Democratic Senator Mark Warner said Trump's "taunting tweets" to Iran did not help de-escalate tensions, a stated goal of the Trump administration.

Kashmiri Shia Muslims shout anti-American and anti-Israel slogans during a protest against the U.S. airstrike in Iraq that killed Soleimani on Friday. (Mukhtar Khan/The Associated Press)

In a series of Twitter warnings to Iran on Saturday, Trump said Washington had identified 52 targets, including some "important to Iran & the Iranian culture," that would be hit hard and fast if Tehran attacked Americans or U.S. assets.

Warner, the top Democrat on the Senate Intelligence Committee, also said he was not yet convinced there was an imminent threat that justified the strike.

"I accept the notion that there was a real threat. The question of how imminent is something that I need more information on," Warner told NBC's Meet the Press.

"We do not need this president either bumbling or impulsively getting us into a major war," added Senate Democratic leader, Chuck Schumer, on ABC's This Week.

Pompeo said intelligence justifying the Soleimani strike has been shared with leaders in Congress and he expects they will be briefed again this week. He bolstered Trump's promise to respond forcefully to any Iranian attack.

The U.S. is sending more than 3,000 troops to the Middle East as tensions rise in the region. (Capt. Robyn Haake/U.S. Army/AFP/Getty Images)

"We've told the Iranian regime: enough. You can't get away with using proxy forces and think your homeland will be safe and secure. We're going to respond against the actual decision-makers - the people who are causing this threat from the Islamic Republic of Iran," Pompeo said.

In a statement on Saturday, House of Representatives Speaker Nancy Pelosi, a Democrat, said the strike was "provocative, escalatory and disproportionate." She said a classified notification lawmakers received on Saturday prompted "serious and urgent questions about the timing, manner and justification of the administration's decision to engage in hostilities against Iran."

White House national security adviser Robert O'Brien said on Saturday the operation was legal and that Justice Department lawyers had signed off on it.

EU crisis meeting

Meanwhile, German foreign minister Heiko Maas called on Sunday for a crisis meeting of his European Union counterparts this week to discuss escalating tension in the Middle East.

"As Europeans, we have tried and tested and resilient channels of communication on all sides, which we must make full use of in this situation," Maas said in a statement.

Maas has proposed to EU foreign affairs chief Josep Borrell that a meeting of the bloc's foreign ministers be brought forward to this week to agree on a common approach.

Maas also said Germany would speak to the Iraqi government after the country's parliament on Sunday backed a recommendation by the prime minister that all foreign troops should be ordered out.

"Our overriding interest is that Iraq's stability and unity should not fall victim to the recent escalation," he said.

Iranian mourners gather around the coffin of Soleiman in Ahvaz, Iran on Sunday. (Hossein Mersadi/Fars news agency/WANA/Reuters)

Germany has around 120 troops deployed in Iraq under the U.S.-led Operation Inherent Resolve.

"We are ready to continue our support if it is desired and the situation allows it," Maas said.

"We are now discussing this intensively with our partners, in the NATO Council, in the European Union, in the anti-IS coalition, and above all with our contacts in Iraq."

NATO ambassadors are also expected to gather on Monday in Brussels for an urgent meeting convened by the head of the military alliance to discuss the situation in the region, a NATO official said.

NATO ambassadors meet regularly, but Monday's meeting was organized at short notice after the alliance decided on Saturday to suspend its training mission in Iraq over security risks following the killing of Soleimani.

Iraq orders foreign troops out

While nations are looking at retaining their military forces in the area, Iraq's parliament on Sunday backed a recommendation by the prime minister that all foreign troops should be ordered out.

A special session passed a resolution saying that the Shia-led government should cancel its request for assistance from a U.S.-led coalition.

On Sunday, Iraq's parliament backed a recommendation by Iraqi Prime Minister Adel Abdul Mahdi that all foreign troops should be ordered out of the country. (Khalid al-Mousily/Reuters)

"Despite the internal and external difficulties that we might face, it remains best for Iraq on principle and practically," said caretaker premier Adel Abdul Mahdi, who resigned in November amid street protests.

He later told France's foreign minister that Iraqi officials were working on implementing the resolution.

Rival Sunni Muslim leaders, including ones opposed to Iranian influence, have united since then in calling for the expulsion of U.S. troops, and Abdul Mahdi's eventual successor is almost certain to take the same view.

However, one Sunni Muslim lawmaker said Sunni Arab and Kurdish minorities fear the expulsion of the U.S.-led coalition will leave Iraq vulnerable to an insurgency, undermine security, and further empower its Iranian-backed Shia militias.

Most Sunni and Kurdish lawmakers boycotted the session, and the 168 lawmakers present were just three more than the quorum.

Qais al-Khazali, leader of the militant group Asaib Ahl al-Haq, said if U.S. troops do not leave Iraq, they would be considered an occupying force. (Alaa al-Marjani/Reuters)

While Iran-backed Iraqi militia commander Qais al-Khazali said Sunday that if U.S. troops do not leave Iraq, they would be considered an occupying force.

The U.S. was disappointed in the decision by Iraq's parliament on Sunday, the State Department said.

"While we await further clarification on the legal nature and impact of today's resolution, we strongly urge Iraqi leaders to reconsider the importance of the ongoing economic and security relationship between the two countries and the continued presence of the Global Coalition to Defeat ISIS," spokesperson Morgan Ortagus said in a statement.