The first major test of U.S. post-war influence in Iraq is now raging over efforts to stop Iran from funneling arms to Syria through Iraqi airspace, but the Iraqis are either unwilling or unable to assure the United States the shipments will cease.

Last week, the Washington Times reported that the Iraqi government was refusing to halt Iranian cargo flights to Syria that fly over Iraqi airspace, despite the fact that U.S. officials believe the flights carry massive and illegal shipments of arms to aid President Bashar al-Assad's regime, which is murdering civilians by the thousands in its struggle to keep power. Publicly, Iraqi Prime Minister Nouri al-Maliki has stated the shipments contain "humanitarian goods, not weapons." However, U.S. officials aren't buying that excuse, and have been repeatedly pressing Maliki behind the scenes to make Iran halt the arms shipments, with limited if any success.

One U.S. official told The Cable that there have been 10 to 20 flights from Iran to Syria with suspected illicit weapons stores on board. Another U.S. official said the resupplies take place via the use of Syrian Air Ilyushin 76 strategic airlifters, similar in size to the Boeing C-17, and that U.S. intelligence reports suspect that the planes are carrying mortar rounds, small arms, ammunition, rockets, and light anti-aircraft guns, which can also be used to fire on people.

Iran's interest in bolstering the Assad regime -- its most important ally in the Arab world -- is clear. CENTCOM commander Gen. James Mattis told Congress earlier this month that the downfall of the Assad regime would be "the biggest strategic setback for Iran in 25 years."

The main U.S. officials involved in the effort to press Iraq on the flights include Vice President Joseph Biden and U.S. Ambassador to Iraq Jim Jeffries, who is expected to step down in the coming weeks and be replaced by Brett McGurk, a lawyer who served on President George W. Bush's National Security Staff and as President Barack Obama's special advisor. Jeffries has met with Maliki several times over the past few weeks on the issue, with the latest meeting occurring this week, two U.S. officials confirmed to The Cable.

Biden personally raised the issue with Maliki in a March 12 phone call, two officials confirmed. Biden told Maliki to make the Iranian over flights a "high priority," and Maliki gave Biden an explicit assurance that he would raise the issue with the Iranians. Three days later, Maliki's office issued a public statement denying that Iran was using Iraqi airspace to transfer weapons to Syria.

At a March 21 meeting of the U.N. committee that monitors enforcement of sanctions against Iran, the U.S. representative condemned Iran's ongoing policy of shipping arms to Syria, although she did not mention the Iraqi role.

"We are alarmed that a majority of the violations reported to the committee involved illicit transfers of arms and related material from Iran to Syria, where the Assad regime is using them to violently repress the Syrian people," said deputy U.S. ambassador Rosemary DiCarlo.

Despite Maliki's unconstructive public statements, one U.S. official involved in the issue told The Cable that the flights seem to have stopped over the last few days.

"The Iraqis have raised this repeatedly and in all kinds of ways with the Iranians and, we believe, with the Syrians. We've seen some changes in over flight patterns," the official said. "There have been cancellations of flights."

The official acknowledged that there was no way to know how long the flights would be stopped, or if they would resume at some later date.  "In the long run, we're all dead," the official said.

This U.S. official argued that the Iraqi government is acting in relatively good faith, and the United States must be sensitive to Maliki's weak position and his need to keep good relations with Iran. Maliki couldn't enforce a ban on Iranian over flights in Iraqi airspace even if he really wanted to, the official said, because Iraq doesn't have an air force and U.S. air assets have all left the country.

"The Iraqi government is trying to cooperate. They have a limited air picture and they have no way to force aircraft down," said the official.

Inside the administration, other officials see the Iraqi government's behavior as distinctly less cooperative, and they believe the Maliki government often tells its U.S. interlocutors exactly what they want to hear -- and then does something completely different.

"On the surface the Maliki government seems to be aligned with us on Syria, but in reality they view a follow-on regime as a serious threat. Maliki's interests are aligned with Iran on this issue," another official said. "This is the opportunity for the Iraqis to push back against Iranian influence and stand with the rest of the region in opposing the killing of innocent civilians, but they're just not doing it."

The Iraqi government has an added incentive to be on its best behavior, as Baghdad is set to host the Arab League summit on March 29. It will be the first such summit in Iraq since the fall of Saddam Hussein, and Maliki is hoping to use the event to symbolize Iraq's return to stability following the U.S. military withdrawal. After the summit, there is less pressure on Maliki to seem sympathetic to the Western and Arab League's position on Syria.

Some inside the administration see Iran's arming of the Syrian regime as particularly troubling, because the Obama administration has decided not to arm the Syrian rebels for now. .

"We've made the decision not to arm the opposition while our adversaries are actively and purposefully rearming the regime and in some cases enhancing the Assad government's ability to crack down on the opposition," the official said. "Would the regime be so cohesive if they weren't being resupplied by the Iranians?"

For some officials and experts, the issue highlights the steep decline of U.S. leverage in Iraq following the military withdrawal.

"This is another sign that the U.S. has lost a tremendous amount of influence inside of Iraq.  We're leaning on Maliki heavily but he's just not cooperating," said Ken Pollack, senior fellow at the Brookings Institution's Saban Center. "Maliki's not looking gratuitously to piss us off, but at the end of the day Iran is wielding a lot more influence in Iraq than we are."

Pollack said that Maliki is walking a tightrope, trying to show the Arabs he will play ball on Syria. Meanwhile, the Iraqi premier knows he can't afford politically to alienate Iran, nor can he throw his lot in with the Sunni Arabs countries, who may never accept him no matter what he does.

"Maliki is not a lover of Iran. He's actually quite suspicious of them and in his way quite an Arab and Iraqi nationalist," said Pollack. "That said, it was the Iranians who ultimately brokered his reelection to the prime ministership, and Maliki understands that Iran wields a lot of influence in Iraq."

Pollak said Maliki probably will continue to make strong statements about the over flights in public and to the Americans, but privately tell Iran they can continue. As long as U.S policy on Syria remains unclear, Pollack argued, the United States can expect nervous partners like Iraq to equivocate.

"The Obama administration hasn't figured out what it wants to do about Syria," he said. "It's hard to make a judgment that we need to invest a whole lot of political capital in getting the Iraqis to turn this off if we don't know what we are doing ourselves."

"To one degree or another, the Iraqis will always support Iran. They are not Israel," one U.S. official said. "What Maliki really believes... nobody knows."