US President Barack Obama has announced that up to 275 military personnel will be deployed to Iraq after fighters from the Islamic State of Iraq and the Levant (ISIL) seized control of the country's north.
The troops will provide support and security for US personnel and the country's embassy in the capital, Baghdad.
"This force is deploying for the purpose of protecting US citizens and property, if necessary, and is equipped for combat," Obama said in a letter to US legislators.
"This force will remain in Iraq until the security situation becomes such that it is no longer needed."
Obama said he was notifying Congress under the War Powers Resolution.
The US was also considering drone strikes in Iraq and has sent more ships to the Gulf, as Sunni fighters threatened to march on Baghdad.
John Kerry, US secretary of state, said on Monday that drone strikes were "not the whole answer" to the ISIL's campaign but added they they could be "one of the options that are important".
"When you have people murdering, assassinating in these mass massacres, you have to stop that. And you do what you need to do if you need to try to stop it from the air or otherwise."
The UN meanwhile, has said it was pulling staff from Baghdad, with at least 58 already moved to Jordan. The organisation planned to relocate others to Erbil, which is the capital of the semi-autonomous Kurdish region of Iraq.
James Bays, Al Jazeera's diplomatic correspondent, said the announcement came only a few days after the UN said it did not believe that Baghdad was at risk of attack by the ISIL.
Carrier strike group
Chuck Hagel, US defence secretary, has also ordered the amphibious transport ship USS Mesa Verde to the Gulf.
The Mesa Verde can carry up to 800 soldiers, their equipment and aircraft such as the Osprey helicopter/plane hybrid and Sea Knight helicopters.
John Kirby, the Pentagon's spokesman, said the Mesa Verde had already joined up with the carrier strike group led by the aircraft carrier, the USS George HW Bush.
AP news agency, citing three anonymous US officials, reported that the White House was considering sending a small number of US special forces into Iraq to slow down the rebel advance.
|US prepares for possible Iraq air strikes
Al Jazeera's Bays said the Iraq crisis was expected to be discussed with Iran on the sidelines of the nuclear talks in Vienna on Monday.
"The Iranian foreign minister will be there, as well as the US deputy secretary of state," he said.
"Iran and the US are deeply concerned about Iraq."
However, the US defence department said that there were no plans for military coordination with Iran in Iraq.
On Monday, ISIL fighters captured Tal Afar, a strategic city along the highway to Syria, moving closer to their goal of linking areas under their control on both sides of the border.
A resident in Tal Afar, said that the rebels in pick-up vans mounted with machine guns and flying black ISIL banners were on the streets, as gunfire rang out.
AP news agency reported that an army helicopter was shot down during clashes with fighters over Saqlawiya village in Anbar province, according to officials.
Al Jazeera's Hoda Abdel-Hamid, reporting from the northern town of Khazer, said Tal Afar represented one of the last strongholds for the Baghdad government.
The city falls a week after rebels captured Iraq's second-largest city, Mosul, and Tikrit.