Militants led by ISIS - the Islamic State in Iraq and the Levant - 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.
The US says it may use drone strikes to halt the militants' advance.
"They're not the whole answer, but they may well be one of the options that are important," said US Secretary of State John Kerry.
The Pentagon said US officials were also open to holding direct talks with Iran over Iraq, but there was "no plan to co-ordinate military activity" between the two countries.
Iranian President Hassan Rouhani earlier said he would consider co-operation if the US took action. » | Monday, June 16, 2014
