John Kerry has made an unannounced visit to Iraq where he spoke to Prime Minister Nuri al Maliki of his concern about Iranian flights over Iraq carrying arms to Syria.

The US Secretary of State also urged Iraq's Sunni Muslim, Shi'ite and ethnic Kurdish factions to commit to the political process as the country's precarious intercommunal balance comes under growing strain from the conflict in neighbouring Syria.

A US official said Washington believes flights and overland transfers from Iran to Syria via Iraq take place nearly every day, helping President Bashar al Assad crush a two-year-old revolt against his rule.

Mr Kerry told Mr Maliki the Iranian flights through Iraqi airspace were "problematic".

"Anything that supports President Assad is problematic," Kerry said during yesterday's visit. "I made it very clear to the prime minister that the overflights from Iran ... are in fact helping to sustain President Assad and his regime."

Abbas al-Bayati, a member of the Security and Defence committee in parliament, said: "We have done our duty by randomly inspecting a number of Iranian flights and we did not find any leaked or smuggled weapons.

"If the US is keen to push us to do more they have to give us the information that they have relating to this," he said.