The first of 300 US military advisers are setting up in Baghdad to help the Iraqi army tackle an expanding Sunni rebellion that has swept the country's north and west.
The special forces troops landed in the Iraqi capital on Tuesday and were beginning evaluations on Wednesday, as Pentagon officials said the US was also increasing surveillance operations in the north of Iraq.
Rear Admiral John Kirby, a Pentagon spokesman, said the soldiers would not be "rushing to the rescue" of Iraqi troops and would not be involved in combat.
Kirby added that the US would carry out bombing raids if it was called upon and was already conducting "manned and unmanned" surveillance flights, the AFP news agency reported.
Al Jazeera's Imran Khan, reporting from Baghdad, said the troops would be based at a camp near Baghdad International Airport.
Iraqi sources were reported to have expressed disappointment that the US did not send equipment in addition to the advisers, Khan said.
Also on Wednesday, Iraqi State TV broadcasted video claiming to show Iraqi troops in control of the oil refinery at Baiji, amid contesting claims as to who was in control there.
The footage, shot by a journalist sympathetic to the government, shows an army helicoper briefly landing at the site before leaving.
Khan said that the video, which the government said was shot on Tuesday, seemed to suggest Iraqi troops were in control of at least part of the refinery.
The Iraqi government would have been hesitant to send a journalist to the area if it wasn't confident it was clear of rebels, Khan said.
The footage was released a day after the US secretary of state, John Kerry, held talks with Kurdish leaders to get them to support a new unity government.
The president of the Kurdistan Regional Government, Massoud Barzani, however stated that Iraq was facing "a new reality and a new Iraq".
The Kurds have taken control of the city of Kirkuk after fighting with Sunni rebels led by the Islamic State of Iraq and the Levant.
The UN on Tuesday said that ISIL's lightning advance in the north of Iraq had killed at least 1,075 people.
A UN spokesman said at least 757 civilians were killed in Nineveh, Diyala and Salaheddin provinces from June 5 to June 22, adding that the figures were "very much a minimum".
The death toll included some civilians, police and soldiers, who had been summarily executed after laying down arms.
A further 318 people were killed in Baghdad and in southern Iraq.