The Obama administration has called on Iraq to require Iranian planes transiting its airspace en route to Syria to land for inspections to prove they are not carrying weapons to Syrian military forces.

Administration officials said Wednesday that they are certain that weapons and other military equipment are aboard the planes, in violation of U.N. Security Council resolutions prohibiting the export of Iranian weapons.

“They are permitting overflights to deliver weapons . . . to support regime forces” of Syrian President Bashar al-Assad, said an official who spoke on the condition of anonymity about the escalating U.S.-Iraqi tension.

An Iraqi spokesman said his government has assurances from Iran that the cargo includes only humanitarian aid and that the United States should provide evidence if it believes otherwise. Iran is Assad’s principal backer in the region.

Iraq’s Shiite-led government, which rejected a recent Arab League call for Assad to resign, has said it does not want to be drawn into a sectarian war in which majority Sunnis make up the bulk of the rebel forces fighting to oust Assad. Assad’s Alawite sect is a subgroup within Syria’s minority Shiite community.

“All nations have a responsibility . . . to seek to prevent the export of Iranian arms,” U.S. National Security Council spokesman Tommy Vietor said. “Iraq has taken steps in the past to meet that obligation and it must continue to do so.”

Three U.S. senators who met with Iraqi Prime Minister Nouri al-Maliki on Wednesday said they told him that he risked damaging relations with the United States, as well as losing U.S. aid under a 2008 U.S.-Iraqi agreement signed before the final withdrawal of U.S. troops last year, the Associated Press reported from Baghdad.

“Bottom line, this kind of problem with these Iranian overflights can make it more difficult to proceed with the [agreement] in the manner that the prime minister and we would like to see happen,” Sen. Joseph I. Lieberman (I-Conn.) told reporters after the meeting.

Lieberman is on a fact-finding trip to Iraq, along with Republican Sens. John McCain (Ariz.) and Lindsey O. Graham (S.C.). All three have called on President Obama to provide U.S. arms and take other military measures in support of the Syrian opposition.

Graham said Obama’s “amazing lack of leadership” had left Iraq “in a pickle” in terms of choosing sides in Syria. “The reason they’re probably not pushing back on Iran is because they don’t see how this ends,” he said of the Iraqis.

The administration, which has long been concerned about Shiite Iran’s influence in Iraq, has a less benign view of Baghdad’s motives but is reluctant to provoke a public breach. While officials expressed sympathy for Iraq’s limited air defense capability to stop the flights even if it wanted to, and for Iraq’s concerns about sectarian spillover from the 18-month-old Syrian conflict, officials said they are determined to enforce Iran sanctions.

In 2007, the U.N. Security Council unanimously passed a resolution prohibiting Iran from supplying or selling any weapons abroad. A further resolution in 2010 “recommended” that all states inspect cargo coming to or from Iran.

Obama raised the issue of the overflights with Maliki after they were first detected early last spring.

At the time, administration officials said they believed the flights contained eavesdropping and electronic jamming equipment, along with technicians and trainers from Iran’s Revolutionary Guard force. Officials said the overflights stopped after the U.S. interventions.

Resumption of the flights was first reported Wednesday by the New York Times, which said they had started again in July.

Vice President Biden, who has been the administration’s primary interlocutor with Maliki, contacted the Iraqi leader last month and promised to provide evidence of weapons shipments, the AP reported. A high-level State Department delegation also visited Baghdad earlier this week.

“Until now, there is no evidence of any violation in this regard, and if anyone has evidence, they should bring it to us and we will take the needed measures,” Ali al-Moussawi, media adviser to Maliki, told the AP.

Asked Wednesday what the administration expected the Iraqis to do, the State Department’s acting deputy spokesman, Patrick Ventrell, said that “the easiest way, we think, is for them to require these aircraft to land and be inspected.”

Ventrell said that “there are international agreements in terms of how aircraft transiting airspace are required to comply with local requirements.” Asked if the United States was offering to help the Iraqis enforce those agreements, he noted that total control of Iraqi airspace was handed over to Iraq by departing U.S. forces.

The State Department also announced Wednesday an additional $21 million in humanitarian assistance to “conflict-affected people inside Syria” and to Syrian refugees in neighboring countries, bringing the total U.S. contribution since last year to more than $100 million.

The U.N. refu­gee agency said this week that refu­gee flows reached 100,000 in August, with the total number of Syrians seeking refuge in Turkey, Jordan, Iraq and Lebanon now about 230,000.