UNITED NATIONS, United States (AFP) - UN Secretary-General Ban Ki Moon urged the international community on Wednesday to unite behind Iraq as a lightening offensive by heavily armed jihadists swept closer to Baghdad.

"The secretary-general urges the international community to unite in showing solidarity with Iraq as it confronts this serious security challenge," his spokesman said in a statement.

The UN chief demanded "full respect for international humanitarian law and human rights law in efforts to counter terrorism and violence in Iraq," spokesman Stephane Dujarric added.

Mr Ban strongly condemned the surge in violence and warned that "terrorism must not be allowed to succeed in undoing the path toward democracy in Iraq." Since the Islamic State of Iraq and the Levant began its spectacular assault in Mosul late Monday, militants have captured a large swathe of northern and north-central Iraq, prompting as many as half a million people to flee their homes.

The speed with which ISIL and its allies have advanced after their seizure on Tuesday of Mosul - a city of two million people - has sent alarm bells ringing in Western capitals.