Iraqi troops at the Iraqi-Syrian border
Iraqi troops at the Iraqi-Syrian borderReuters

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 told CNN on Monday.

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, said 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 on Saturday that "there have been reports in the public space about rockets being fired at American forces on bases in Iraq."

"So we've seen a little bit of an uptick there. And that's, again, another indicator for us of Iran reaching out," he added, speaking at an event hosted by the Reagan National Defense Forum in California.

In the most recent attack on Monday, four rockets struck a military training site near Baghdad International Airport where US troops train members of the Iraqi Counter Terrorism Service. While no US personnel were injured or killed, six Iraq security force members were injured according to a statement from the Iraqi military's media office.

Following the attack, Iraqi security forces searched the area and confiscated a pick-up truck near the base carrying a rocket launcher, according to the same statement.

Another attack took place this past Thursday, with rockets landing near Balad Air Base.

The types of rockets involved in some of the attacks have become increasingly sophisticated, one official told CNN on Monday.

The attack on the airport and a December 3 attack on Al Assad air base both involved 122 mm rockets. Previous attacks used a 107mm variant, which has a shorter range and carries less explosive power than the 122 mm rocket. The 122 mm rocks are more dangerous and can be fired from a greater range and are launched from sophisticated improvised rail systems.

The increasingly sophisticated attacks come amid US accusations that Iran has been moving short-range ballistic missiles into Iraq.

A report last year said that Iran had transferred short-range ballistic missiles to its Shiite allies inside Iraq.

Iran rejected the report, claiming it aimed to harm Iran’s ties with its neighbors. Iraq’s Foreign Ministry said the article was “without evidence”, though it stopped short of denying its contents.

Meanwhile, multiple defense officials have told CNN that the US is weighing deploying 4,000-7,000 additional troops to the Middle East in the face of the increased threat from Iran.

Esper pushed back on notion that additional troops are going to be sent, saying Saturday, "I'm not looking at any major deployments coming up in the region. That said, on a day-to-day basis, we monitor what's happening in the Middle East, on the Korean peninsula, in the European theater -- all over the world, and we make adjustments to our forces up or down based on what the needs of the commander are, and that happens again, routinely."

In October, Esper indicated that all of the nearly 1,000 troops withdrawing from northern Syria are expected to move to western Iraq to continue the campaign against Islamic State (ISIS) and “to help defend Iraq.”