A Katyusha rocket has hit inside Baghdad’s Green Zone as loud explosion was heard but no casualties were reported by the Iraqi security services.

Alert sirens sounded briefly in Baghdad after the explosion was heard, according to Associated Press reporters on the east side of the Tigris River.

The cause of the explosions is not yet known. No group has yet claimed responsibility.

Iraqi security forces have closed down the Green Zone neighborhood.

A police source told AFP that "initial reports indicate that the rocket was fired from an open field" in southern Baghdad.

All US bases in Iraq have been put on standby amid reports that the rocket has hit an area close to the US embassy.

"A low-grade rocket did land within the International Zone near the US Embassy and the JOC (Joint Operation Center). There were no casualties or significant damage; no US-inhabited facility was impacted," the US State Department said in reaction to the incident.

”We are in close and continuing contact with senior Iraqi officials regarding this incident and investigating the circumstances. At present there are no claims of responsibility," it added.

"As noted, there have been no claims of responsibility and no US-inhabited facility was struck. But, we take this incident very seriously. We have made clear over the past two weeks and again underscore that attacks on US personnel and facilities will not be tolerated and will be responded to in a decisive manner," the State Department said.

It also said the US "hold Iran responsible" if any such attacks are conducted by its allied forces and their elements in Iraq, and "will respond to Iran accordingly.”

Shortly after the rocket blast, US President Donald Trump posted another anti-Iran tweet, claiming that if Tehran wants a war, "that will be the official end of Iran."