The US military on Thursday said it was building up its ground-based air defence systems to protect its troops in Iraq but would not reveal when they are ready for service.
“The establishment of ground-based air defences in Iraq continues, but for operational security reasons we are not providing status updates as those systems come online,” said Capt Bill Urban, spokesman for the US Central Command.
The build-up follows a series of attacks on Iraqi bases hosting US members of the international coalition against ISIS.
Washington has placed the blame on Iran or the Iraqi militias it backs.
Capt Urban said the Iraqi government was “well aware of our collective need for air defence protection of service members within Iraq, and we continue to co-ordinate closely with our Iraqi counterparts".
US President Donald Trump said on Wednesday that his administration had information that "Iran or its proxies are planning a sneak attack on US troops and/or assets in Iraq".
"If this happens, Iran will pay a very heavy price indeed," Mr Trump said on Twitter.
A battery of the US Patriot air-defence system was delivered to Ain Al Assad base in western Iraq last week and is now being assembled.
Another was shipped to Erbil, the capital of Iraq’s semi-autonomous Kurdistan region, US and Iraqi sources told AFP.
American coalition troops were moved to several bases after withdrawing from four in Iraq in the past month.
The coalition said the moves were pre-planned and not connected to the attacks on bases housing foreign troops.
Capt Urban said on Thursday that the attacks, which killed or injured coalition and Iraqi service members, "violate Iraqi sovereignty".
“The US and our coalition partners remain committed to the lasting defeat of ISIS and supporting Iraq's long-term security and stability,” he said.