A series of attacks on US personnel in Iraq and Syria in recent days, have deeply concerned the Pentagon on Thursday.

US diplomats and troops in Iraq and Syria were targeted in three rocket and drone attacks on Wednesday alone, including at least 14 rockets hitting an Iraqi air base hosting US forces, injuring two American service members.

Analysts believed the attacks were part of a campaign by Iranian-backed militias, although there were no immediate claims of responsibility.

“They are using lethal weaponry. I don’t know how you can say anything other than it is a serious threat,” Pentagon spokesman John Kirby told reporters.

Iraqi militia groups aligned with Iran vowed to retaliate after US strikes on the Iraqi-Syrian border killed four of their members last month.

Iran denied supporting attacks on US forces in Iraq and Syria and condemned US air strikes on Iranian-backed groups.

The United States has been holding indirect talks with Iran aimed at bringing both nations back into compliance with the 2015 Iran nuclear deal, which was abandoned by then-President Donald Trump. No date has been set for a next round of the talks, which adjourned on June 20.