The Pentagon said on Thursday it was deeply concerned about a series of attacks on US personnel in Iraq and Syria in recent days.

"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, according to Reuters.

On Wednesday, two rockets were fired at the US embassy inside Baghdad's Green Zone.

The embassy's anti-rocket system diverted one of the rockets and the second rocket fell near the zone's perimeter.

That attack came a day after an explosive-laden drone attacked Erbil airport in northern Iraq. The drone reportedly targeted the US base on the airport grounds.

On Monday, a drone was intercepted and shot down near the US embassy in Baghdad. Sources said the drone failed to reach the embassy compound. There were no injuries.

While there were no immediate claims of responsibility for the attacks, analysts believed they were part of a campaign by Iranian-backed militias.

Rocket attacks have regularly targeted Iraqi bases as well as the fortified Green Zone in Baghdad, where the US embassy is located, since the US elimination of top Iranian General Qassem Soleimani last January.

Last week, the US launched air strikes against three targets it said were used by pro-Iran groups in eastern Syria and western Iraq.

A day later, pro-Iranian militias fired several shells at a US base in eastern Syria's Al-Omar oil field, causing damage but no casualties.