US interests in Iraq are seeing regular bombardment by rockets and drones, in an escalation which saw 17 rockets rain down on US assets in the space of 24 hours.
Fourteen were aimed at the Al Assad airbase on Wednesday evening and three at the US embassy in Baghdad's heavily fortified Green Zone.
The Iraqi Security Media Cell blamed "outlaws" for Thursday's attacks, which injured two US personnel.
"The first rocket landed near the headquarters of the National Security Agency, the second in the Grand Festivities Square and another rocket landed near Sheikh Omar area in a residential neighbourhood," the Cell wrote on Twitter about one of the attacks.
Spokesman for Operation Inherent Resolve to fight against ISIS Col Wayne Marotto said the attacks "undermine the authority of Iraqi institutions, the rule of law and Iraqi National Sovereignty".
Thursday's incidents were the latest in stepped-up attacks on US facilities including the Ain Al Assad military base in Iraq's Anbar province and an oil field in eastern Syria.
No armed groups have claimed responsibility for the attacks, but experts say Iran-aligned militias could be behind them.
In June, four Iran-backed Iraqi militia group members were killed by US strikes on the Iraqi-Syrian border. The Pentagon said the facilities struck were used by Iran-backed militia groups to support strikes in Iraq.
Commander of Kataib Sayyid Al Shuhada, Abu Alaa Al Walae, vowed to retaliate.
In an interview with the Associated Press on Wednesday, Mr Al Walae said his group's operations will have "everyone" talking about them.
“We want an operation that befits those martyrs,” the commander said in reference to the four dead militiamen.
“We want it to be an operation in which everyone says they have taken revenge on the Americans."
An almost daily bombardment of facilities housing US troops and personnel has ensued, but the group has not claimed responsibility.
On Tuesday, the Al Omar oil field in Syria’s eastern border with Iraq was targeted. The US-backed Kurdish Syrian Democratic Forces which guard the facility said it thwarted the attack on Syria's largest oilfield.
The facility did not sustain any damage as a result of the attack.
The oil field was also targeted some 10 days ago, but US troops stationed there managed to escape without injury.
Al Assad Air Base
On Wednesday, the Al Assad Air Base and Erbil's airport were both targeted in what spokesman for the Iraqi Ministry of Defence Yehia Rasool called a "terrorist attack". He repeated Iraq's position and refusal to be used as a battleground for a "proxy war" between US and Iranian forces.
A truck carrying a rocket launcher was also found near the air base. It is believed to have been used for the attacks.
Shortly after the Ain Al Assad attack, three rockets were fired at the US Embassy in Iraq’s capital prompting sirens to blare in warning in Baghdad’s Green Zone which hosts a number of government buildings and embassies.
The embassy itself was not hit, the Iraqi army said, but three nearby locations in the heavily fortified area were.
"One rocket landed near the headquarters of the National Security Agency, the second in a public square and the third on the outskirts of a residential area damaging a resident's car", the Iraqi military said.
A co-ordinated escalation
Associate fellow at the Washington Institute Hamdi Malik called recent events part of a "co-ordinated escalation" among Iran-backed militia.
Speaking to Reuters, he said the attacks reflect the first time targets were simultaneously hit in Syria and Iraq. Ongoing talks in Vienna to revive the Joint Comprehensive Plan of Action (JCPOA) known as the Iran nuclear deal have stalled, after Iranian officials said they were expecting to have positive results by the time Iran elects a new president.
Cleric Ebrahim Raisi won the vote in June.
"It seems to me they have the green light from Iran to escalate, especially given that the nuclear negotiations are not going well. But at the same time, they do not want to escalate beyond a certain point ... and they don’t want to overcomplicate the negotiations Iran is holding with the West," Mr Malik said.
Iran-backed militias are heavily concentrated west of the Euphrates River in Deir Al Zor province. Hundreds of US troops remain in the vicinity of Syria's main oil fields which are controlled by US-backed forces.
- With additional reporting from Reuters and Associated Press