President Barack Obama has said he will send about 200 more US troops to Iraq to protect Americans and the US embassy in Baghdad amid fierce fighting in the country between government forces and Sunni armed groups.
The US is also sending a detachment of helicopters and drone aircraft.
Pentagon spokesman Rear Admiral John Kirby said on Monday about 200 forces arrived in Iraq on Sunday to reinforce security at the US embassy, its support facilities and the Baghdad International Airport.
Another 100 personnel were also due to move to Baghdad to "provide security and logistics support."
"These forces are separate and apart from the up to 300 personnel the president authorised to establish two joint operations centres and conduct an assessment of how the US can provide additional support to Iraq's security forces," Kirby said in a statement.
Of the initial deployment to the embassy of 275 troops earlier this month, 100 had been on standby outside the country, but are now moving into positions in Baghdad, the Pentagon said.
The announcement will bring to nearly 800 the total number of US forces in and around Iraq to train local forces, secure the embassy and protect Washington's interests.
President Obama has ruled out sending combat troops back into Iraq. But he says the additional troops will be equipped for combat.
Obama says the troops will stay in Iraq until security improves so that the reinforcements are no longer needed.
The new troop movement is part of the Obama administration's attempt to help Iraqi Prime Minister Nouri al-Maliki's push back the stunning gains that fighters from the Islamic State in Syria and the Levant (ISIL) have made over the last few weeks.
A US official, speaking on condition of anonymity, said that the United States was also considering putting up a new joint military operations centre in the northwest of Iraq's semi-autonomous Kurdish region.
While no final decisions have been made, the official said that the new operations centre, which would be the second such cell the US has established since Iraq's security deteriorated earlier this month, could be placed in the province of Duhok, in Iraq's farthest northern reaches near Syria and Turkey.
US soldiers at a similar joint operations centre in Baghdad are gathering information about the situation on the ground and overseeing US soldiers who are taking stock of the Iraqi military in the field.
In addition to supplying weaponry and conducting surveillance flights, Washington has sent hundreds of military advisers and other soldiers to assess the Iraqi army, which largely evaporated in northern Iraq when ISIL fighters swept in earlier this month.
President Obama has not ruled out air strikes against ISIL, which has gained strength as the war in neighboring Syria has dragged on.