Iran has resumed shipping military equipment to Syria over Iraqi airspace to save its ally President Bashar al-Assad, senior American officials tell the New York Times.

The Obama administration pressed the Shia-dominated Iraqi government of Prime Minister Nuri al-Maliki of Iraq to shut down the air corridor, but as the Syrian rebels gained ground, Iran doubled down in supporting Assad. The flights started up again in July and have continued despite protests to Baghdad from American officials.

Military experts tell the Times the flights have let Iran to supply the Assad regime despite the rebels' efforts 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.