The drone, which was launched at the Erbil air base by an Iranian-backed militia before sunrise on Oct. 26, penetrated U.S. air defenses and crashed into the second floor of the barracks housing American troops at about 5 a.m, according to two U.S. officials familiar with the matter.

But the device laden with explosives failed to detonate and in the end only one service member suffered a concussion from the impact, said the officials, who asked to remain anonymous to speak freely about the attack. The U.S. had got lucky, they added, as the drone could have caused carnage had it exploded.

The incident was among at least 40 separate drone and rocket attacks that have been launched at U.S. forces by Iranian-backed militias in Iraq and Syria over the past three weeks in response to American support for Israel in the Gaza war, according to Pentagon data and the two U.S. officials.

The bombardment has only caused a few dozen minor injuries so far, with many of the rockets and one-way attack drones intercepted by U.S. air defenses in Iraq and Syria, where a total of 3,400 American troops are based.

David Schenker, a former U.S. assistant secretary of state at the Washington Institute for Near East Policy think-tank, cautioned that while neither Iran and its allied groups nor the U.S. appeared to want a direct confrontation, the risks were growing. The possibility of a major strike that draws America into a conflict is "a very realistic concern," he said.

"I think they are calibrating the attacks to harass rather than kill en masse U.S. troops," he said of Iraqi and Syrian militias. "But there's a lot more they can do."

It's unclear how President Joe Biden would respond to a major attack that kills a large number of Americans. Struggling in opinion polls ahead of next year's presidential election, Biden has so far sought to limit the U.S. role in the conflict mostly to ensuring military aid to Israel.

The war broke out when gunmen from Hamas - the Iranian-funded militant group that rules the Palestinian enclave of Gaza - burst into southern Israel on Oct. 7, killing 1,400 people, mostly civilians, and taking more than 240 hostages. Since then, Israel has bombarded the coastal territory relentlessly, killing more than 10,000 people, many of them children.

Iran says it had no role in Hamas' Oct. 7 raid on Israel, though it has welcomed the attack.

On Sunday, U.S. Secretary of State Antony Blinken flew to Iraq - where most of the attacks on U.S. forces have taken place - to push Iraqi Prime Minister Mohammed Shia Al-Sudani to crack down on the militias operating there and avert any escalation.

Yet Sudani has had little luck in persuading the militia groups from letting up their assault, or convincing their bankrollers in Iran to rein them in, according to five senior lawmakers in Sudani's governing coalition, a security adviser to the premier and a militia commander.

The prime minister and around 10 senior members of his government met with the commanders of about a dozen militia groups in Baghdad on Oct. 23 to press the groups to halt their attacks on U.S. forces, said the seven people, who were either present or were briefed on the meeting.

The plea largely fell on deaf ears, though, with most of the commanders vowing to keep up their assault until Israeli forces ended their siege and bombardment of the Gaza Strip, they added.

"No one - not the prime minister or anyone else - can stand against our religious duty," said Ali Turki, a Shi'ite lawmaker in the governing coalition as well as a commander with the powerful Iranian-backed Asaib Ahl al-Haq militia.

Arif al-Hamami, another Shi'ite lawmaker, said the prospects for diplomacy looked bleak: "I don't think that the prime minister has the power to stop the attacks as long as Israel is committing atrocities in Gaza with American help."

The Iraqi and Iranian governments didn't immediately respond to requests for comment on the militia attacks and the risk of escalation.

IRAQ LEADER'S APPEAL TO IRAN

Iraq's prime minister has limited control over the militias, whose support he needed to win power a year ago and now form a powerful bloc in his governing coalition. The militant groups, which proliferated in Iraq in the wake of the U.S.-led invasion in 2003 that toppled Saddam Hussein and his Sunni government, are trained and funded by Shi'ite power Iran.

For Sudani, it's been a case of shuttle-diplomacy.

Hours after meeting Blinken on Sunday, the premier flew to Tehran to directly appeal to Supreme Leader Ayatollah Ali Khamenei and other Iranian officials for help, according to a senior Iraqi politician close to the prime minister who was briefed on the visit.

Sudani asked the Iranian officials to pressure the militias into halting their attacks on U.S. forces in Iraq, fearing his politically and economically unstable country could ill afford an escalation that would see the Americans strike back against the militants, the politician said.

The officials told him that the militias in Iraq made their own decisions and Tehran wouldn't interfere in the situation there, the politician added.

Iran has decried the retaliatory Israeli assault on Gaza as a genocide and warned that if it isn't halted, the U.S. will not be "spared from this fire." Meanwhile, the Hezbollah movement backed by Tehran in Lebanon - a group that sources say have acquired powerful Russian anti-ship missiles - has warned Washington that it would pay a heavy price in a regional war.

'LAUGHING AT US IN TEHRAN'

Biden faces his own dilemmas as he receives a steady stream of reports about hostilities in the region. Among attacks outside Iraq and Syria in recent weeks, Iranian-aligned Houthi fighters unleashed 15 drones and four cruise missiles off the coast of Yemen that were shot down by U.S. Navy destroyer with a crew of hundreds of sailors, U.S. military officials say.

The present crisis has erupted following years of steady U.S. withdrawal of military assets from the Middle East, including air defenses, as Washington seeks to focus on Russia's invasion in Ukraine and mounting tensions with China. That refocus accelerated after Biden's complete pullout from Afghanistan and the Taliban's takeover there two years ago.

The response by Biden has been cautious so far; he ordered overnight strikes on two Iranian-linked arms storage facilities in Syria last month while they were unoccupied, but has not ordered any strikes in Iraq. On Wednesday, Biden followed up with a similar strike in Syria and U.S. Defense Secretary Lloyd Austin cautioned: "We urge against any escalation."

Biden has warned Iranian-backed groups across the region, including the large Hezbollah movement in Lebanon, against expanding the conflict but he and other officials have declined to be explicit about what they would do in response.

The U.S. hopes a military show of force will dissuade any serious attack and has deployed two aircraft carrier strike groups and even taken the rare step over the weekend of announcing that an Ohio-class submarine had moved to the region.

Beyond sending air defenses like the Patriot system and a high-altitude system, the U.S. military is also taking additional steps to protect its tens of thousands of troops in the region, according to officials.

The measures include beefing up security at U.S. military bases in the region by increasing patrols, restricting access and boosting intelligence gathering, they said.

The response to the crisis from Biden, a Democrat, hasn't been strong enough for many of his critics, including Republicans in Congress.

"They are laughing at us in Tehran," said Republican Senator Tom Cotton, a member of the Senate Armed Services Committee. "Iran will continue to target Americans until President Biden gets serious about imposing severe costs on Iran."

At a hearing with Austin on Oct. 31, Republican Senator Lindsey Graham repeatedly asked if the deaths of U.S. service members would trigger a direct response against Iran. Austin demurred, only saying that Iran should be held "accountable."

"I wish you would be more clear, because if one of these soldiers is killed..." Graham said, pausing for effect.

For some, the recent attacks on U.S. troops stir painful memories of the massive truck bomb in Beirut that shredded a Marine barracks, killing 241 U.S. service members, 40 years ago last month. The United States holds Hezbollah responsible for the suicide bombing though the group has denied involvement.

David Madaras was a 22-year-old Marine when the concussive wave hit him from the explosion in 1983. As he recalls digging through the rubble where some of his friends were buried, he sees modern-day parallels that make him uneasy.

"We had rocket attacks, mortar attacks, before we got hit with the big bomb," he said. "Does history repeat itself?"