WASHINGTON — Iran has resumed shipping military equipment to Syria over Iraqi airspace in a new effort to bolster the embattled government of President Bashar Assad of Syria, according to senior US 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 Nouri al-Maliki of Iraq. But as Syrian rebels have gained ground and 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 US officials have continued ever since.

Military specialists say 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 Joe Biden, who has played the lead role on Iraq policy for the Obama administration, discussed the Syrian crisis in a phone call with Maliki on Aug. 17. The White House has declined to disclose details, but a US official who would not speak on the record said that Biden had registered his concerns over the flights.

The Obama administration pressed Iraq to shut down the air corridor Iran had been using.

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 Assad’s government in power.

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

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 Assad as a development that might strengthen his Sunni Arab and Kurdish rivals in the region. Some states that are the most anxious to see Assad go, like Saudi Arabia, Qatar, and Turkey, have poor relations with Maliki and his Shi’ite-dominated government.

Iran has a big 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.

As part of Iran’s assistance to the Assad government, it has provided Syrian authorities with the training and technology to intercept communications and monitor the Internet, according to US officials. Iranian Quds Force personnel, they say, have been involved in training the heavily Alawite paramilitary forces the government has increasingly relied on, as well as Syrian forces that secure the nation’s air bases.

The Iranians have even provided a cargo plane that the Syrian military can use to ferry men and supplies around the country, according to two US officials.

In a new twist, according to one US official, there have been reliable reports that Iraqi Shi’ite militia fighters, long backed by Iran during its efforts to shape events inside Iraq, are now making their way to Syria to help the Assad government.

While they have not specifically discussed the assistance it is airlifting to Syria, US officials have spoken publicly about Iran’s involvement there. ‘‘Iran is playing a larger role in Syria in many ways,’’ Defense Secretary Leon E. Panetta said last month. ‘‘There’s now an indication that they’re trying to develop, or trying to train, a militia within Syria to fight on behalf of the regime.’’

David Cohen, a senior Treasury Department official on terrorism issues, said last month that Hezbollah has been training Syrian government personnel and has facilitated the training of Syrian forces by Iran’s Quds Force.