The US Secretary of State John Kerry says drone strikes "may well be" an option to stop an assault by militants in Iraq.
Earlier, officials said Sunni militants seized the northern Iraqi city of Tal Afar.
Militants led by ISIS - the Islamic State in Iraq and the Levant - (pictured) captured key cities including Mosul and Tikrit last week, but some towns were retaken.
UN Humanitarian Chief Valerie Amos said there were reports of "major human rights violations", including summary executions of civilians.
British Foreign Secretary William Hague says the UK is not planning any military involvement in Iraq, but is considering what humanitarian aid it can offer.
It's also reported the US may also consider direct talks with Iran about the issue.
Former UK special representative to Iraq, Sir Jeremy Greenstock, says Iranian forces may be able to help in ways American troops won't:
