FILE - in this Tuesday, Feb. 13, 2018 file photo, an Iraqi army soldier stand guard near a U.S.- made Iraqi Air Force F-16 fighter jet at the Balad Air Base, Iraq.
FILE - An Iraqi army soldier stands guard near a U.S.- made Iraqi air force F-16 jet at Balad Air Base, Iraq, Feb. 13, 2018 . Three rockets fired by militants hit the base June 14, 2019, in one of several attacks Iraqi authorities are investigating.

The Iraqi army on Wednesday said it was investigating a series of mysterious rocket attacks on military bases hosting U.S. personnel and an oil field linked to the U.S. oil giant ExxonMobil.  
"The Joint Operations Command has charged all the intelligence agencies to gather information and identify those behind the rocket and missile fire at a number of military and civilian sites in Baghdad and other provinces," the Iraqi Security Media Cell said in a statement, adding that the security forces were taking measures to deter attacks.   
"Security forces ... will strike with an iron fist all of those who spread insecurity, fear and anxiety and carry out an agenda that conflicts with Iraq's national interests," the army's media office said. 
The Iraqi government statement came as an unidentified rocket early Wednesday hit near the operations headquarters of ExxonMobil in the southern province of Basra.   
The Iraqi Security Media Cell said a Katyusha rocket landed in al-Barjasiya district west of Basra. It did not disclose information about the perpetrators but said three workers were injured in the attack. 

After the attack, ExxonMobil evacuated 20 foreign employees, according to Sky News Arabia, citing unidentified local officials. 
This was the fifth time since late last week that rockets reportedly had struck near installations that house U.S. personnel.  Last Friday, the Iraqi Defense Ministry said three rockets hit Balad air base in Saladin province, followed by a rocket attack on the same day in al-Jadryah district near the U.S. Embassy. The Iraqi Security Media Cell reported similar attacks on al-Taji military base in northern Baghdad on Monday and Mosul's Presidential Palace on Thursday. Iraqi security officials said the attacks have caused no casualties or major damage. 
Iran-backed militias 
No group has claimed responsibility for the recent attacks, but U.S. forces in the past have been targeted by the Islamic State and Iran-backed militants within the Popular Mobilization Forces (PMF).  
The PMF is an umbrella organization of several Shiite militias formed in 2014 after the Iraqi army fled the area following IS attacks. The group includes U.S. terror-designated militias, such as Asa'ib Ahl al-Haq and the Badr organization, and Iran-friendly parties such as the Islamic Supreme Council of Iraq. 
U.S. officials consider the PMF a regional proxy of Iran that is increasingly threatening and provoking American troops in Iraq and Syria. 
An assessment of the U.S. anti-IS operations by the Pentagon's Office of the Inspector General last November reported that Iran-backed militias were most likely behind two attacks targeting U.S. facilities in Iraq in September, including mortar attacks that targeted Baghdad's Green Zone and landed near the U.S. Embassy, and rocket attacks that targeted the Basra airport, near the U.S. Consulate. 
Last month, the U.S. State Department ordered a partial evacuation of the U.S. Embassy in Baghdad after receiving intelligence about "Iranian activity" that put American facilities and personnel at "substantial risk."