a serious danger to the region," Mladenov told AFP.
"Iraq faces the biggest threat to its sovereignty and territorial integrity" in years.
The violence has stoked regional tensions, with Iraq accusing neighbouring Saudi Arabia today of "siding with terrorism" and financing the militants.
The comments came a day after the Sunni kingdom blamed "sectarian" policies by Iraq's Shiite-led government for triggering the unrest.
The prime minister of Iraq's autonomous Kurdish region told the BBC it would be "almost impossible" for the country to return to how it was before the offensive, and called for Sunni Arabs to be granted an autonomous region of their own.
Alarmed by the collapse of much of the security forces in the face of the militant advance, foreign governments have begun pulling out diplomatic staff.
US President Barack Obama announced that around 275 military personnel "equipped for combat" were being deployed to Iraq to help protect the embassy in Baghdad and assist US nationals.
Washington has already deployed an aircraft carrier to the Gulf, but Obama has ruled out a return to combat in Iraq for US soldiers. As the US weighed its next move, Secretary of State John Kerry said that drone strikes could be used.
Washington has ruled out cooperating militarily with Tehran, but the two governments -- which have been bitter foes for more than 30 years -- held "brief discussions" on the crisis in Vienna.
Drones have been used by the US against militants in Afghanistan, Pakistan and Yemen, but have been criticised by human rights groups for their heavy civilian toll.
Doubts are growing that the Iraqi security forces can hold back the militant tide, despite military commanders trumpeting a counter-offensive.
Soldiers and police fled en masse as the insurgents swept into Iraq's second city of Mosul a week ago, abandoning their vehicles and uniforms.
The jihadists are said to have killed scores of Iraqi soldiers as they pushed their advance, including in a "horrifying" massacre in Salaheddin province that has drawn international condemnation.
Top Shiite cleric Grand Ayatollah Ali al-Sistani has called for volunteers to join the battle against the militants and thousands have signed up.