Secretary of State John Kerry made an unannounced visit to Iraq on Sunday and said he told Prime Minister Nuri al-Maliki of his concern about Iranian flights over Iraq carrying arms to Syria.
Washington believes such flights and overland transfers take place nearly every day and help Syrian President Bashar al-Assad in his efforts to crush a two-year-old revolt against his rule, said a U.S. official, who spoke on condition of anonymity.
Kerry said he had told Maliki the Iranian flights through Iraqi airspace were "problematic".
"Anything that supports President Assad is problematic," Kerry told reporters. "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."
Speaking before the meeting, the U.S. official said the Iraqi government had inspected only two flights since last July and that Kerry would argue Iraq did not deserve a role in talks about neighboring Syria's future unless it tried to stop the suspected arms flow.
Iraqi officials denied allowing the transfer of weapons from Iran to Syria through Iraqi airspace. Abbas al-Bayati, a member of the Security and Defence parliamentary committee, 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 U.S. is keen to push us to do more they have to give us the information that they have relating to this," he said.
More than a decade after the U.S.-led invasion that toppled Saddam Hussein, Iraq still struggles with insurgents, sectarian friction and political feuds among Shi'ite, Sunni and Kurdish factions who share power in the government of Shi'ite premier Maliki.
Sunni Islamist insurgents linked to al Qaeda and invigorated by the war next door in Syria - where Sunni rebels are battling Assad, an ally of Shi'ite Iran - are regaining ground in Iraq and have stepped up attacks on Shi'ite targets in recent months in an attempt to provoke a wider sectarian confrontation.
Kerry held talks with representatives of all three communities, including Osama al-Nujaifi, the Sunni speaker of parliament.
He also spoke by telephone to Massoud Barzani, president of Iraq's Kurdish region, whose regional government is pressing ahead with plans to build an oil pipeline to Turkey that Washington fears could lead to the break-up of Iraq.
According to reporters at a picture-taking session at the start of Kerry's talks with Maliki, the U.S. diplomat appeared to joke that Hillary Clinton, his predecessor, had said Iraq would do whatever Washington asked.
"The Secretary told me that you're going to do everything that I say," Kerry said, according to the reporters.
"We won't do it," Maliki, also joking, replied, the reporters said.
In his talks with Maliki, Kerry also asked the Iraqi prime minister and his cabinet to reconsider a decision to postpone local elections in two Sunni-majority provinces, Anbar and Nineveh, the U.S. official said.
The Iraqi cabinet last week postponed the votes, which were due on April 20, for up to six months because of threats to electoral workers and violence there - a step Washington believes will only increase tensions.
While violence has fallen from the height of the sectarian slaughter that killed tens of thousands in 2006-2007, insurgents have carried out at least one major attack a month since U.S. forces left. Bombings and killings still happen daily, often aimed at Shi'ite areas and local security forces.
More than a dozen car bombs and suicide blasts tore through Shi'ite Muslim districts in the Iraqi capital Baghdad and other areas on Tuesday, killing nearly 60 people on the 10th anniversary of the U.S.-led invasion that ousted Saddam.
Further complicating security, thousands of Sunni protesters have rallied in Anbar against Maliki, whose Shi'ite-led government they accuse of marginalizing their minority sect since the fall of Sunni strongman Saddam.