Iraq said on Saturday it will step up searches of Iranian flights via its airspace to Syria, days after US Secretary of State John Kerry publicly criticised Baghdad for turning a blind eye to them.

But while Prime Minister Nuri al-Maliki's spokesman spoke of newly tightened restrictions on Iranian flights to Syria, the head of Iraq's civil aviation authority acknowledged that no planes had been searched since October.

"Because of a lot of information which referred to transportation of weapons, we have increased the activity of inspections," Maliki's spokesman Ali Mussawi told AFP.

"We will carry out more random searches, to be assured that there is no weapons transfer."

Asked if the move was in response to Kerry's comments last Sunday during a surprise visit to Baghdad, Mussawi replied: "No one has provided us with evidence -- just information."

Kerry had told reporters while in Baghdad that he "made very clear to the prime minister that the overflights from Iran are in fact helping to sustain President (Bashar) al-Assad and his regime."

He told Maliki that American politicians were "watching what Iraq is doing" and noted that anything that helped Assad was "problematic"

For months, Washington has accused Baghdad of turning a blind eye as Tehran sends military equipment through Iraqi airspace, and has called on authorities to make random, unannounced inspections.

"They are suspending their disbelief, looking the other way, and averting their gaze," a US official said, speaking on condition of anonymity.

Iraq announced two inspections of aircraft, both in October, but the New York Times reported in December that Iran appears to have been tipped off by Iraqi officials as to when inspections would be conducted, helping Tehran avoid detection.

Nasser Bandar, the head of Iraq's civil aviation authority, told AFP that no planes had been searched since October because "we have not seen any suspicious flights since then."

"We will inspect the flights we have suspicions about," he added.

Bandar said for the past several days there had been no cargo flights from Iran bound for Syria, and that air traffic between the two countries had been limited to passenger aircraft, which are not searched.

Iran has remained a steadfast ally of Assad's regime despite the conflict in his country which according to the United Nations has killed more than 70,000 people since it erupted in March 2011.