British Foreign Secretary William Hague made a surprise visit to Baghdad on Thursday to urge leaders to unite in the face of a Sunni militant offensive that threatens Iraq's existence.

Hague is due to meet with several leaders over the militant offensive, which he said poses "a mortal threat to the stability and territorial integrity of Iraq," according to a Foreign Office statement.

"The Iraqi state is facing an existential threat, with huge ramifications for the future stability and freedom of this country," he was quoted as saying.

"The single most important factor that will determine whether or not Iraq overcomes this challenge is political unity."

His visit follows an unannounced trip by US Secretary of State John Kerry in which he pushed a similar message, urging Iraqi leaders to unite against the Sunni militant onslaught, which has been led by the jihadist Islamic State of Iraq and the Levant, or ISIL.

The insurgents have overrun vast swathes of five provinces north and west of Baghdad in an offensive that has alarmed the international community, left more than 1,000 people dead and displaced hundreds of thousands.