The US Department of State's Rewards for Justice programme said Thursday it was offering a reward of up to $3 million for information on attacks against Americans in Iraq.

The announcement comes a day after an attack was carried out with three "explosive-laden" drones on Baghdad airport, where US troops are deployed.

"O faithful people of Iraq, cowardly terrorists are attacking US diplomatic missions in Iraq, then they are fleeing to hide among civilians," said a statement in Arabic on the Twitter account of Rewards for Justice.

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"America is offering a reward of up to $3 million for information on planned attacks or past ones against American diplomatic installations," said the statement, which was accompanied by a video.

It provided a US telephone number, and said the information could be sent via the messaging apps Whatsapp, Telegram or Signal.

US interests in Iraq have come under repeated attacks since October 2019, including with rockets, with the United States routinely blaming them on Iran-backed factions.

Since the beginning of the year, a total of 42 attacks have targeted the US embassy in Baghdad, Iraqi bases housing US troops or Iraqi convoys carrying logistical support.

The latest attack on Wednesday was carried out with three drones packed with explosives, the Iraqi army said on Thursday.

It said one of the drones had been intercepted by air defences Wednesday evening, the fourth such drone attack in less than two months.

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Experts say the use of such drones marks an escalation in attacks against American interests by pro-Iranian forces.

The techniques are similar to those deployed by Huthi rebels in Yemen against Tehran's regional rival Saudi Arabia.

Wednesday's attack was the first such attack on targets in the Iraqi capital, the Arab world's second-most populated city.

On Wednesday, five rockets also landed at Balad, an airbase further north where American contractors are based, a security source said.

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They did not cause any casualties or damage, the source said.

The Balad base has been targeted so regularly that US weapons firm Lockheed Martin withdrew last month, citing concerns about the safety of its personnel.

Pro-Iran groups on Wednesday had hailed what they described as "one more victory" for the state-affiliated Hashed al-Shaabi paramilitary coalition, as commander Qassem Muslah was released.

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Muslah had been arrested in May by police intelligence on suspicion of ordering the killing of Ihab al-Wazni, a pro-democracy activist shot dead earlier that month by unidentified gunmen on motorbikes.

Iraqi authorities have repeatedly blamed "outlaws" of carrying out "terrorist" attacks with rockets or explosive-laded drones but have struggled to identify those behind these assaults.