Washington (CNN) The US government believes that Iran is behind a series of increasingly sophisticated rocket attacks on joint US-Iraq military facilities in Iraq, several US officials tell CNN. The attacks have taken place as the US has grown increasingly concerned that Iran may be planning new provocations against US troops and interests in the region.

The US military strongly believes Iranian-backed groups inside Iraq are responsible according to a US official with direct knowledge of the recent incidents.

There have been nine rocket attacks on or in the vicinity of Iraqi facilities that host US troops in the last five weeks with the most recent one taking place on Monday.

"We take these incidents seriously as do our Iraqi Security Forces partners, who are investigating these events. We have made clear that attacks on US. and Coalition personnel and facilities will not be tolerated and we retain the right to defend ourselves. US forces operate in Iraq at the invitation of the government of Iraq to support Iraqi forces against ISIS," Pentagon spokesperson Cdr. Sean Robertson told CNN in a statement.

Secretary of Defense Mark Esper said Saturday that "there have been reports in the public space about rockets being fired at American forces on bases in Iraq."

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