Pentagon officials believe that the more than a dozen missiles launched at two bases in Iraq housing U.S. forces Tuesday night were designed to kill Americans, but the Iranian efforts were thwarted by the military's Early Warning Systems, Defense Secretary Mark Esper and Gen. Mark Milley, chairman of the Joint Chiefs of Staff, told reporters on Wednesday.

“I believe based on what I saw and what I know is that they were intended to cause structural damage, destroy vehicles and equipment and aircraft and to kill personnel," Milley said.

U.S. military officials were warned about Iran's pending ballistic missile strike "when the air defense systems went active," he said.

Milley added that “a little bit of early warning” and “normal defensive procedures” at Al-Assad Air Base prevented American troops from being killed in the missile strike.

US MILITARY HAD ADVANCE WARNING OF IRANIAN BALLISTIC MISSILE ATTACK, OFFICIALS SAY

“In my estimation from what I know now, I think it has more to do with the defensive techniques that our forces used as opposed to intent,” Milley said, adding that U.S. forces "took sufficient defensive measures.”

Speaking to reporters after a returning from a briefing with Congress, Esper said that the 16 short-range ballistic missiles (originally estimated to be 15), were fired from three locations inside Iran during the attack early Wednesday local time, and were intended to avenge the death of Iranian Quds Force Gen. Qassem Soleimani, who was killed in a U.S.-led drone strike at Baghdad International Airport last week.

Milley said that between 1,000- to 2,000-pound warheads were sitting on top of the Iranian missiles fired into Iraq.

The defense secretary also said he believes the U.S. has reestablished deterrence from another Iranian attack but added that he expects to be challenged by Shiite proxy militias in the region.

Esper confirmed that the missiles hit two military bases -- the Al-Assad Air Base and a separate military base in Erbil, which house U.S. forces in Iraq. Eleven of those missiles hit Al-Assad and one hit Erbil. Four of the missiles malfunctioned and failed to hit their target.

The damage at the bases was largely contained to “tentage, taxiways, the parking lot, damaged helicopter, things like that, nothing I would describe as major,” Esper said.

Pentagon officials met with President Trump twice in the past 24 hours following the attack, Esper said. Trump announced earlier Wednesday in a nine-minute address to the nation that he would be imposing even more sanctions on Iran in response to the attack.

Democrats on Capitol Hill criticized Trump's initial decision to take out Soelimani without first consulting Congress or allies in the region, which has led to escalating tensions in the Middle East, and have since demanded answers from the administration about the alleged imminent threat that sparked the Soleimani attack, and how Trump will de-escalate the tensions with Iran and keep Americans safe.

House Speaker Nancy Pelosi announced plans for the House to vote Thursday on a War Powers Resolution that aims to limit Trump's military action toward Iran, saying Democrats have “serious, urgent concerns about the administration’s decision to engage in hostilities against Iran and about its lack of strategy moving forward.”

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Lawmakers on both sides of the aisle expressed concerns coming out of Wednesday's Iran briefing about the administration bypassing Congress' approval for military escalations.

“Most members of Congress do not have access to the intelligence that I think is the most compelling. That’s simply the nature of the intelligence and it’s restricted the Gang of 8,” Esper said, referring to the term for the bipartisan group of eight lawmakers who are briefed on classified intelligence matters.

Fox News' Lucas Tomlinson and Jennifer Griffen contributed to this report.