Iran launched ballistic missiles at bases in Iraq housing U.S. military personnel on Tuesday, marking the latest escalation in a spiraling conflict between Washington and Tehran that is threatening to embroil the United States in a new war in the Middle East.

The strikes followed Iranian threats of retaliation for the U.S. drone strike in Baghdad early Friday, authorized by President Donald Trump, that killed Quds Force leader Qasem Soleimani, a major military figure in Iran who oversaw the nation's operations outside its borders.

Soleimani's killing has also plunged relations between the United States and Iraq into crisis, with Iraqi politicians calling for all foreign troops to depart. The United States has had a force of about 5,000 in Iraq in recent years as part of the mission against the Islamic State extremist group.

It wasn't immediately clear whether Iran's attack on Tuesday resulted in injuries or deaths on the military installations targeted in Iraq. According to the Pentagon, the missiles were fired directly from Iranian territory at U.S. forces - a state-on-state attack that departs from past actions Iran has taken against U.S. forces through the use of proxy militias.

Trump didn't immediately comment on the attacks. In a statement, White House press secretary Stephanie Grisham said: "We are aware of the reports of attacks on US facilities in Iraq. The President has been briefed and is monitoring the situation closely and consulting with his national security team."

House Speaker Nancy Pelosi, D-Calif., said in a statement on Twitter that she was "closely monitoring the situation following bombings targeting U.S. troops in Iraq."

"We must ensure the safety of our servicemembers, including ending needless provocations from the Administration and demanding that Iran cease its violence," Pelosi said. "America & world cannot afford war."

Pentagon spokesman Jonathan Rath Hoffman said in a statement that Iran had launched more than a dozen ballistic missiles against U.S. military and coalition forces in Iraq.

"It is clear that these missiles were launched from Iran and targeted at least two Iraqi military bases hosting U.S. military and coalition personnel," at al-Asad and Irbil, Hoffman said. "We are working on initial battle damage assessments."

Hoffman said that the bases had been on high alert as a result of Iranian threats, and that the Defense Department had "taken all appropriate measures to safeguard our personnel and partners."

"As we evaluate the situation and our response, we will take all necessary measures to protect and defend U.S. personnel, partners, and allies in the region," Hoffman added.

American forces remain on heightened alert throughout the region. Military officials have sent close to 10,000 additional troops to the Middle East in recent days as tensions with Iran have risen.

The Iranian strikes followed a stampede at the funeral for Soleimani that left dozens dead and delayed the procession for several hours before he was buried in his home city of Kerman.

The Trump administration has defended its killing of the Iranian general, saying he was responsible for the deaths of hundreds of U.S. service members by Iranian proxy militias during the Iraq War.

Secretary of State Mike Pompeo and Defense Secretary Mark Esper have said the U.S. military was acting on intelligence showing that Soleimani was making plans for additional attacks against Americans when he arrived in Baghdad for meetings with Iraqi officials.

His car, which was also carrying a senior Iraqi militia figure, was targeted just outside Baghdad International Airport.

Iran's ballistic missile attacks on Tuesday are the latest episode in an escalating conflict between Washington and Tehran.

In 2018, Trump withdrew the United States from an international nuclear deal with Iran, calling the pact poorly negotiated by the Obama administration.

Instead, Trump opted for a "maximum pressure" campaign against Tehran, which included the reapplication of harsh sanctions. Trump administration officials said the measures were designed to bring Iran to the table for a new nuclear deal and curb broader malign behavior by Tehran in the Middle East, including its support of Houthi rebels in Yemen.

Iran, in response, began lashing out last year, seizing and attacking tankers in the Strait of Hormuz. The Trump administration attributed to Iran a major attack on Saudi oil installations in the fall that briefly led to a 50 percent reduction in the kingdom's oil production.

In using ballistic missiles, Iran relied on what the Defense Department considers one of its three core capabilities. In a briefing held in November, a senior defense analyst focused on Iran told reporters at the Pentagon that the missiles constituted a primary component of Tehran's strategic deterrent.

"Lacking a modern air force, Iran has embraced ballistic missiles as a long-range strike capability to dissuade its adversaries from attacking Iran," said the official, Christian Saunders. "Iran also has the largest missile force in the Middle East, with substantial inventory of close-range ballistic missiles, short-range ballistic missiles and medium-range ballistic missiles that can strike targets throughout the region as far as 2,000 kilometers away."

The Pentagon assessed that Iran will deploy an increasing number of "more accurate and lethal ballistic missiles" and continue to improve its existing missile inventory while fielding new land-attack cruise missiles.

This article was written by Dan Lamothe and Paul Sonne, reporters for The Washington Post.