US Secretary of State John Kerry has made a surprise visit to Iraq to urge its leaders to do more to prevent planes from neighbouring Iran flying through its airspace to supply arms and ammunition to the Syrian government, and to discuss the sectarian conflict still plaguing the country ten years after the US ousted the Saddam Hussein regime.

Kerry will meet Iraqi Prime Minister Nouri al-Maliki, and also plans a telephone conference with the head of the Kurdish regional government, Massoud Barzani, to address pressing issues.

Last year, former secretary of state Hillary Clinton secured a promise from the Iraqi government that it would monitor and inspect Iranian flights passing over Iraq en route to Syria. A senior US official claimed that the government has failed to keep its promise as only two aircraft have been inspected since.

A source also alleged that Iranian aircraft cross Iraq "close to daily", according to reports.

Iran says shipments reaching Syria are meant to provide humanitarian aid for Syrians embroiled in the two-year civil war between Bashar al-Assad's government and Sunni-dominated rebels.

Kerry will urge al-Maliki to prevent Iranian aircraft from flying over Iraq. Kerry is also expected to warn the Iraqi prime minister of the dire consequences of letting Iran gradually establish a foothold in Syria.

He will also address the sectarian differences between the Iraqi government and the Kurds.

Last May, the semi-autonomous state of Iraqi Kurdistan based in Irbil announced an agreement with Turkey to build new oil and gas pipelines, which angered the Iraqi government in Baghdad.

Kerry will insist that Barzani maintain the unity of Iraq and stress that Kurdistan will not be able to survive financially without the support of Baghdad.

Kerry will also suggest that the Iraqi government reassess its decision to delay provincial elections in Anbar and Nineveh.

This is Kerry's first visit to Iraq since becoming US secretary of state earlier this year.