BAGHDAD – Just days after the 10th anniversary of the U.S. invasion of Iraq, U.S. Secretary of State John Kerry confronted Baghdad for continuing to grant Iran access to its airspace and said Iraq’s behavior was raising questions about its reliability as a partner.

Speaking to reporters during a previously unannounced trip to Baghdad, Kerry said that he and Iraqi Prime Minister Nouri al-Maliki had engaged in “a very spirited discussion” on the Iranian flights, which U.S. officials believe are ferrying weapons and fighters intended for the embattled Syrian government.

Kerry said the plane shipments — along with materiel being trucked across Iraqi territory from Iran to Syria — were helping President Bashar Assad’s regime cling to power by increasing their ability to strike at Syrian rebels and opposition figures demanding Assad’s ouster.

“I made it very clear that for those of us who are engaged in an effort to see President Assad step down and to see a democratic process take hold … anything that supports President Assad is problematic,” Kerry said at a news conference at the U.S. Embassy in Baghdad after meeting separately with Maliki at his office. “And 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.”

The overflights in Iraq have long been a source of contention between the U.S. and Iraq. Iraq and Iran claim the flights are carrying humanitarian goods, but American officials say they are confident that the planes are being used to arm the support of the Assad regime. The administration is warning Iraq that unless action is taken, Iraq will be excluded from the international discussion about Syria’s political future.

U.S. officials say that in the absence of a complete ban on flights, Washington would at least like the planes to land and be inspected in Iraq to ensure that they are carrying humanitarian supplies. Former Secretary of State Hillary Rodham Clinton secured a pledge from Iraq to inspect the flights last year, but since then only two aircraft have been checked by Iraqi authorities, according to U.S. officials.

One senior U.S. official traveling with Kerry said the sheer number of overflights, which occur “close to daily,” along with shipments trucked to Syria from Iran through Iraq, was inconsistent with claims they are only carrying humanitarian supplies. The official said it was in Iraq’s interest to prevent the situation in Syria from deteriorating further, particularly as there are fears that al-Qaida-linked extremists may gain a foothold in the country as the Assad regime falters.

The official, who spoke on condition of anonymity because the official was not authorized to speak publicly on the matter, said there are clear links between al-Qaida linked extremists operating in Syria and militants who are also carrying out terrorist attacks in Iraqi territory with increasing regularity.

Kerry’s comments come as U.S. lawmakers are calling for President Obama to do more to stop the bloodshed in Syria, including possible airstrikes against Assad’s aircraft fleet.