WASHINGTON — Iran has resumed shipping military equipment to Syria over Iraqi airspace in a new effort to bolster the embattled government of President Bashar al-Assad of Syria, according to senior American officials.


The Obama administration pressed Iraq to shut down the air corridor that Iran had been using earlier this year, raising the issue with Prime Minister Nuri Kamal al-Maliki of Iraq. But as Syrian rebels gained ground and Mr. Assad’s government was rocked by a bombing that killed several high officials, Iran doubled down in supporting the Syrian leader. The flights started up again in July and, to the frustration of American officials, have continued ever since.


Military experts say that the flights have enabled Iran to provide supplies to the Syrian government despite the efforts Syrian rebels have made to seize several border crossings where Iranian aid has been trucked in.


“The Iranians have no problems in the air, and the Syrian regime still controls the airport,” said a retired Lebanese Army general, Hisham Jaber, who heads the Middle East Center for Studies and Research in Beirut.


Vice President Joseph R. Biden Jr., who has played the lead role on Iraq policy for the Obama administration, discussed the Syrian crisis in a phone call with Mr. Maliki on Aug. 17. The White House has declined to disclose details, but an American official who would not speak on the record said that Mr. Biden had registered his concerns over the flights.


The Iranian flights present searching questions for the United States. The Obama administration has been reluctant to provide arms to the Syrian rebels or establish a no-fly zone over Syria for fear of being drawn deeper into the Syrian conflict. But the aid provided by Iran underscores the reality that Iran has no such hesitancy in providing military supplies and advisers to keep Mr. Assad’s government in power.


And Mr. Maliki’s tolerance of Iran’s use of Iraqi airspace suggests the limits of the Obama administration’s influence in Iraq, despite the American role in toppling Saddam Hussein and ushering in a new government. The American influence also appears limited despite its assertion that it is building a strategic partnership with the Iraqis.


Mr. Maliki has sought to maintain relations with Iran, while the United States has led the international effort to impose sanctions on the Tehran government. At the same time, the Iraqi prime minister appears to look at the potential fall of Mr. Assad as a development that might strengthen his Sunni Arab and Kurdish rivals in the region. Some states that are the most eager to see Mr. Assad go, like Saudi Arabia, Qatar and Turkey, have poor relations with Mr. Maliki and his Shiite-dominated government.


Iraq could take several steps to stop the flights, including insisting that cargo planes that depart from Iran en route to Syria land for inspection in Baghdad or declaring outright that Iraq’s airspace cannot be used for the flights.


Iraq does not have a functioning air force, and since the withdrawal of American forces last December, the United States has no planes stationed in the country. Several airlines have been involved in ferrying the arms, according to American officials, including Mahan Air, a commercial Iranian airline that the United States Treasury Department said last year had ferried men, supplies and money for Iran’s paramilitary Quds Force and Hezbollah, the militant Lebanese group backed by Iran.


One former American official said it was not entirely clear what cargo was being sent to Syria before the flights stopped in March. But because of the type of planes involved, the nature of the carriers and the Iranians’ reluctance to have the planes inspected in Iraq, it was presumed to be tactical military equipment.


At the time the flights were suspended, Iraq was preparing to host the Arab League summit meeting, which brought to Baghdad many leaders opposed to Mr. Assad. Immediately after the meeting, President Obama, in an April 3 call to Mr. Maliki, reinforced the message that the flights should not continue.


Iran has an enormous stake in Syria. It is Iran’s staunchest Arab ally, a nation that borders the Mediterranean and Lebanon, and has provided a channel for Iran’s support to Hezbollah.