The US is considering drone strikes in Iraq and has sent more ships to the Gulf, as the UN said it was pulling its staff out of Baghdad following lightning advances by Sunni fighters in the north.
John Kerry, the secretary of state, on Monday said drone strikes were "not the whole answer" to the Islamic State of Iraq and the Levant'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."
President Barack Obama would on Monday be presented with options to tackle the Iraq crisis, the White House said.
The UN meanwhile 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.
Chuck Hagel, the US defence secretary, has also ordered the amphibious transport, the USS Mesa Verde, to the Gulf.
The Mesa Verde can carry up to 800 Marines, 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.
|US prepares for possible Iraq air strikes
AP news agency, citing three anonymous US officials, reported that the White House is considering sending a small number of US special forces into Iraq to slow down the rebel advance.
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," said Bays. "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.
Rebels make more gains
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 trucks 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 militants 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 fall of Tal Afar comes a week after rebels captured Iraq's second-largest city, Mosul, and Tikrit.