The United States says recent attacks on bases in Iraq and Syria that host American troops are typical of assaults by Iran-backed militia groups.

Pentagon Press Secretary John Kirby said in a briefing on Wednesday that Washington is mindful of the threat environment, and Iran knows how seriously we take the safety and security of our people and our mission in Iraq and Syria.
He admitted that the pace and frequency of such attacks have increased, noting that the reason can possibly be related to the anniversary of the strike that killed Qasem Soleimani and the change in the US mission in Iraq that officially transitioned to an advise and assist mission.
Iran-backed militia groups have said that change in the mission wasn't good enough, and that any presence beyond December 31st would be responded to… So, the result can be a confluence of all those things, Kirby said, stressing that no matter the reasons, the US was “preparing for the possibility of stepped-up attacks at the end of December.”
He added that the US military has increased its focus on the threat of rocket and drone attacks by Iran-back militia groups that “have become over time increasingly precise and increasingly lethal”.