US Defense Secretary Mark Esper warned Thursday "all options are on the table" a day after three US coalition members, including two US troops, were killed in a rocket attack in Iraq.
Coalition forces spokesman Colonel Myles Caggins has confirmed that 15 rockets hit Iraq's Camp Taji base on Wednesday night (local time), and reports indicate there were many injuries.
Esper and Army General Mark Milley, chairman of the Joint Chiefs of Staff, declined to provide any more information about any impending United States retaliation for the attack at Camp Taji north of Baghdad.
The U.S. -led military coalition in Iraq says 18 107-millimetre Katyusha rockets struck Iraq's Taji military camp.
An air strike killed 26 fighters of Iraqi paramilitary group Hashed al-Shaabi in eastern Syria after a deadly attack on US-led coalition troops in Iraq, a war monitor said Thursday.
Shortly afterward, the caretaker premier, Abdul-Mahdi, said he would no longer carry out most of his official duties and called for early elections.
The Pentagon launched attacks against Khataib Hezbollah's weapons caches after American contractor Nawres Hamed was killed in a similar rocket strike on December 27.
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The attack threatens a unsafe escalation, with suspected US-led coalition air strikes promptly targeting Iran-aligned Iraqi fighters in neighboring Syria, the Syrian Observatory for Human Rights (SOHR) said.
In the two months since the killing of powerful Iranian military commander Qasem Soleimani and the retaliatory Iranian strikes, the outbreak of violence many United States officials feared had not materialised, until now.
There have now been four American troops killed in Iraq this year alone. The attack is under investigation by the Coalition and Iraqi Security Forces.
US Secretary of State Mike Pompeo said the attack would "not be tolerated", while Foreign Secretary Dominic Raab said it was a "cowardly" act.
Ben Wallace, Britain's defense secretary, told media outlets on Thursday: "Last night's attack on United Kingdom and coalition personnel was a cowardly and retrograde act".
The military has not confirmed who is responsible for the strike.
The Iraqi parliament voted to expel all foreign soldiers from the country in the wake of the killing of Soleimani, a decision that can only be executed by the government.
On Sunday, two USA soldiers were killed while helping Iraqi forces battle ISIS remnants north of Baghdad.
As Quds Force commander, Soleimani was intimately involved in the training, supplying and controlling of Kataeb Hezbollah and other Iranian proxy forces across the Middle East, and he was highly revered by those groups.