In an address to the nation on January 8, Trump said that "no American or Iraqi lives were lost" in the strikes, "no Americans were harmed," and that the bases suffered "only minimal damage."

Iran says the attack on the Ain al-Asad Air Base and the Harir Air Base was in retaliation for the killing of Islamic Revolutionary Guards Corps (IRGC) Major General Qasem Soleimani.

The Quds Force commander died in a U.S. air strike in Baghdad last week, triggering a major escalation in already deteriorating relations between Tehran and Washington.

World leaders condemned Iran's attack and urged an end to the escalating confrontation between Tehran and Washington.

The attack came hours after the 62-year-old Soleimani, who helped orchestrate Tehran's overseas clandestine and military operations, was buried in his hometown on January 7.

It was described as the most direct assault by Iran on the United States since the seizing of the U.S. Embassy in Tehran in 1979.

U.S. Defense Chief Mark Esper said that Iran launched at least 16 short-range ballistic missiles from at least three locations.

He added that the U.S. military remains "poised and ready" should further attacks occur and said that its forces have restored the necessary "level of deterrence" against Iran.

Just before midnight in Iraq's capital, Baghdad, two rockets fell inside the heavily fortified Green Zone, causing no casualties, the Iraqi military said. The U.S. and other embassies are based in the area, as well as the seat of Iraq's government.

Pro-Iranian factions in Iraq have vowed to avenge Soleimani's death.

Meanwhile, U.S. House of Representatives Speaker Nancy Pelosi said the chamber would move forward with a resolution to "limit the president's military actions regarding Iran."

The resolution "will go to the Rules Committee this evening and will be brought to the floor tomorrow," Pelosi said in a statement, after concerns about the administration's strategy and decisions were not addressed in a briefing with lawmakers.

Trump has previously threatened military action against Iran if it were to target U.S. personnel and bases.

But in his January 8 address, the president said, "The fact that we have this great military and equipment...does not mean we have to use it."

"Iran appears to be standing down, which is a good thing for all parties concerned," he added.

Washington would impose new financial and economic sanctions on Iran, Trump said, adding that they would remain until the country "changed its behavior."

Iran "must abandon its nuclear ambitions and end its support for terrorism," he said.

"The civilized world must send a clear and unified message to the Iranian regime: Your campaign of terror, murder, mayhem will not be tolerated any longer. It will not be allowed to go forward," he continued.

Trump also said the United States was "ready to embrace peace with all who seek it."

Iranian state television said Iran had fired 15 missiles at U.S. targets in Iraq early on January 8.

AP quoted two U.S. officials as saying 10 of the missiles hit the Ain al-Asad Air Base in Iraq's western province of Anbar and one targeted the Harir Air Base in Irbil in Iraq's semiautonomous Kurdish region.

Four failed, according to the officials.

Iranian Supreme Leader Ayatollah Ali Khamenei said the attack was "a slap in the face" delivered to the United States and called for an end to the U.S. presence in the Middle East.

And President Hassan Rohani said that Washington might have "cut off the arm" of Soleimani but America's "leg" in the region would be cut off in response.

The IRGC said the attack was "a total success by all accounts."

Iranian state television claimed, without offering evidence, that the strikes killed "at least 80 terrorist U.S soldiers" and also damaged helicopters, drones, and other equipment.

Iraq's military said there were "no victims among the Iraqi forces."

And a NATO official said that there were no casualties among the personnel on the alliance's training mission in Iraq.

In a statement, the country's presidency condemned the Iranian strikes, reiterating its rejection of breaches of Iraq's sovereignty.

President Barham Salih's office also said that Iraq "has previously declared its refusal to be a starting point for aggression against any country, and also refuses to be a source of threat to any of its neighbors."

Iraqi Prime Minister Adil Abdul-Mahdi received a verbal message from Iran telling him a response to the U.S. killing of Soleimani was either imminent or under way, Mahdi's spokesman earlier said in a statement.

Tehran told Abdul-Mahdi it would only target locations where U.S. forces were present but did not specify the locations, his spokesman said.

The Iraqi prime minister simultaneously received a call from the United States while missiles were falling on the Ain al-Asad and Harir air bases, the spokesman said.

NATO Secretary-General Jens Stoltenberg called on Tehran to "refrain from further violence."

Later in the day, NATO said Stoltenberg had a phone call with Trump, who asked the secretary-general for the military alliance to "become more involved in the Middle East."

The two agreed that NATO "could contribute more to regional stability and the fight against international terrorism," a statement said.

Besides Iraq, NATO conducts training missions in Afghanistan. It is also a member of the U.S.-led coalition against the Islamic State extremist group in Iraq and Syria.

British Prime Minister Boris Johnson has condemned Iran's "reckless and dangerous" missile attacks and called for "urgent de-escalation" by Tehran and Washington.

France said that "the priority is more than ever for a de-escalation," while Germany called on Iran to end a "spiral" of conflict.

EU foreign-policy chief Josep Borrell said, "It is in no one's interest to turn up the spiral of violence even further."

Following their meeting in Istanbul, the leaders of Russia and Turkey issued a joint statement saying that the "exchange of attacks and use of force by any party do not contribute to finding solutions to the complex problems in the Middle East."

Russian President Vladimir Putin and his Turkish counterpart, Recep Tayyip Erdogan, called on all parties to "act with restraint as well as common sense and to prioritize diplomacy."

Separately, Erdogan said Turkey did not want the region to be "a stage for proxy wars."

Turkey's Foreign Minister Mevlut Cavusoglu is to visit Iraq on January 9 as part of diplomatic efforts to "alleviate the escalated tension" in the region, his ministry said.

In a Twitter post, Pakistani Prime Minister Imran Khan also stressed the need to take immediate steps to de-escalate tensions and said he was sending his foreign minister to Iran, Saudi Arabia, and United States to meet with counterparts.

The UN mission in Iraq said the country "should not pay the price for external rivalries."

Meanwhile, the Syrian government expressed solidarity with Iran, saying Tehran had the right to defend itself "in the face of American threats and attacks."

Iranian Defense Minister Amir Hatami told state television that should Washington retaliate, Iran would respond with "proportional" force.

"We used short-range missiles.... I hope this will be a memorable lesson for America," Hatami said.

"Iran's response [to any U.S. retaliation] will be proportional to what America will do," he said, adding that Trump "has turned the [U.S.] administration into a terrorist government."

Several major airlines, including Air France and Germany's Lufthansa, announced they were rerouting flights to avoid airspace over Iraq and Iran, while the Federal Aviation Administration banned U.S. carriers from the area.

Iran's military leaders and pro-Iranian factions in Iraq had vowed to avenge his death.

Professor Stephen David of Johns Hopkins University told Voice of America that while Iran's retaliation wasn't unexpected, it was surprising that Tehran chose to launch the attack from its own territory and use ballistic missiles -- a weapon substantially more powerful than a rocket.

"I think it was surprising that the attack was from Iran. It was not at all deniable," David said. "It was clear what the source of the attack was; the fact they use ballistic missiles also was a bit of a surprise, something of an escalation."

With reporting by AFP, AP, Reuters, and dpa