An explosives-packed drone slammed into Iraq's Arbil airport in the first reported use of such a weapon against a base used by US-led coalition troops in the country, officials said Thursday.
There were no casualties in the strike on the capital of northern Iraq's autonomous Kurdish region late Wednesday, although it did cause damage to a building in the military part of the airport.
It comes after around 20 bomb and rocket attacks blamed on pro-Iran Shiite armed groups against facilities used by coalition troops or diplomats in Iraq since US President Joe Biden took office in January.
The attacks have mostly been claimed by shadowy Shia armed groups aligned with Iran who are demanding the Biden administration set a pullout date for Iraq as it has for Afghanistan.
"A drone packed with TNT targeted a coalition base at Arbil airport," the Kurdish region's interior ministry said.
There was no immediate claim of responsibility for the attack, which caused an explosion heard across Arbil.
But a shadowy pro-Iranian group calling itself Awliyaa al-Dam (Guardians of Blood), which claimed a previous attack on the same airport in February, hailed the strike on the messaging app Telegram.
In the February attack, more than a dozen rockets targeted the military complex inside the airport, killing an Iraqi civilian and a foreign contractor working with US-led troops.
Washington -- which has promised to withdraw the troops it deployed in support of Baghdad's successful fightback against the Islamic State group but has resisted setting a date -- said it was "outraged" by the latest violence.
"The Iraqi people have suffered for far too long from this kind of violence and violation of their sovereignty," State Department spokesman Ned Price tweeted.
- 'Dangerous escalation' -
Leading Kurdish politician, ex-foreign minister Hoshyar Zebari, explicitly blamed pro-Iranian "militia" for the attack.
"It seems the same militia who targeted the airport two months ago are at it again," Zebari tweeted. "This is a clear & dangerous escalation."
A senior US defence official told AFP that while Wednesday's strike marked the first use of a drone to target US troops inside Iraq, Iran's allies in the country had already shown they had the technology.
Washington has said a January drone attack on the Saudi capital Riyadh was carried out from southern Iraq on behalf of Yemen's Iran-backed Houthi rebels.
"The Iranian-backed militias have drones now with a 15-foot (four and a half metre) wingspan," the defence official said.
"It's an Iranian-made CAS-04 which we've already seen weaponized by the Houthis against Saudi."
The official said the technology was constantly being improved.
"They now have the capacity for a rocket-assisted launch. The range is 1,200-1,500 kilometres (750 to 930 miles) if they add fuel tanks to it.
"They can even be loaded onto a ship from (the Iraqi port of) Basra and brought even closer to target. These can be pre-programmed with a GPS destination."
Analysts said the use of drones offered a new way to penetrate US defences.
"Suicide drones are particularly useful in these types of hits as they can avoid counter rocket, artillery and mortar systems," said Hamdi Malik, associate fellow at the Washington Institute.
Pro-Iran groups have been ratcheting up their rhetoric, vowing to ramp up attacks to force out the "occupying" US forces, and there have been almost daily attacks on coalition supply convoys across the mainly Shia south.
Earlier Wednesday, two roadside bombs exploded as convoys passed through the southern provinces of Dhi Qar and Diwaniyah, security sources said.
The United States last week committed to withdraw all remaining combat forces from Iraq, although the two countries did not set a timeline for what would be a second US withdrawal since the 2003 invasion which toppled Saddam Hussein.
The announcement came as the Biden administration resumed a "strategic dialogue" with the government of Prime Minister Mustafa al-Kadhemi, who is seen as too close to Washington by pro-Iranian groups.