Iran provides military support to President Bashar Al Assad by way of regular clandestine flights between Tehran and Damascus, according to a Syrian official with knowledge of air traffic between the two capitals.
Up to three supply flights occur each week between the two cities, none of them appearing on public timetables, said the official, who has access to certain details of air traffic in and out of Damascus International Airport.
The flights typically take place at night, with no weekly schedule set in advance.
“There are private flights every week, sometimes three a week, and they are controlled by an Iranian officer in Damascus,” the Syrian official said. He spoke on condition of anonymity because of the sensitive nature of the information.
“Everyone must follow this [Iranian] man’s orders. We have been told he is the second most important man in Syria and that we are to do as he says without question,” said the official, who continues to support the Assad regime and work for the Syrian government in Damascus.
The identity of the Iranian officer is not publicly known.
“We have not been told openly, but we know the flights are being organised by the [Iranian] Revolutionary Guard,” he said.
The aid flights have been taking place “for months”, the official said. He did not specify the date they began and also said he was not privy to any specific information about what was carried on board the aircraft.
However, another Syrian who has regular access to Damascus International Airport said at least some of the Iranian aircraft had been seen transporting fighters to help regime forces battle rebels.
While Syria and Iran have made no secret of their close alliance, including financial and political links, details of military cooperation are carefully guarded. Iran has denied deploying any army personnel in Syria, while Syrian officials have rejected allegations that the regime is dependent on Iranian muscle in its war against foreign-backed insurgents.
The US, a cautious supporter of Syrian rebels, has long complained about flights between Iran and Syria, saying Tehran airlifts soldiers and weapons to the Assad regime.
Last year, in the final months of her tenure as US secretary of state, Hillary Clinton struck an agreement with Iraq to intercept and search Iranian planes flying through its airspace, a measure that the government in Baghdad – which has close ties to Tehran – has shown little desire to implement.
As of March, only two aircraft passing between Iran and Syria had been searched, US officials said earlier this year, with nothing but humanitarian aid found on board.
In August last year, the US joined Syrian opposition groups in accusing Iran of training pro-Assad militia forces, some of them Syrian but others Shiite volunteers from across the Islamic world, eager to join what they see as a war between Sunnis and Shiites.
The Wall Street Journal reported last month that thousands of Shiite militiamen had received training at a base near Tehran ahead of their deployment to the frontlines of the civil war.
Shiite militia groups, including Lebanon’s Hizbollah, have played a key role in bolstering Syrian regime forces, who have weakened by defections and stretched by a nationwide conflict, most visibly in the battle for Qusayr in April and May.
Pro-regime fighters are moved between Iran and Syria without normal ticketing, immigration and boarding procedures, the Syrian with regular access to Damascus airport said.
The man, who is not involved in the revolt against the Assad regime, described how earlier this month, he watched as dozens of Iran-linked fighters were taken off one of the unregistered flights after a mechanical failure grounded the aircraft at the Damascus International Airport.
Instead of taking off under cover of darkness, the flight was delayed and the passengers were transferred to an alternative civilian aircraft bound for the Iranian capital.
“A green military bus escorted by Syrian soldiers moved all of these men from an unmarked plane on to another plane which was going to Tehran,” the man said.
“It was daylight, it was about 8am, and I could see them clearly - a bus full of men, they were in civilian clothes and didn’t have weapons but they looked like soldiers. There were no women, no children. I was told they were ‘with the Iranians’ and no one was allowed near enough to talk to them,” he said.
Several hours after the flight departed there were angry exchanges between Syrian officials over the way the transfer had been handled, he said, with a senior officer apparently upbraiding junior officials for allowing the flight to leave during normal working hours, while the airport was open to civilian traffic.
A former Syrian military intelligence officer, who broke ranks with the regime more than a year ago but who remains in contact with some current intelligence staff, said he doubted claims that the secret flights were ferrying Iranian fighters to Syria.
“Iran is supporting Syria politically, financially and logistically, but it is not true they are flying three airliners full of fighters each week,” he said, speaking on condition of anonymity.
“There is misunderstanding and a lot of exaggeration about Iranian involvement on the ground in Syria. The truth is that Assad’s army remains powerful, they still have the battalions and the men they need to fight. They really just need money to pay for the war,” he said.
“Iranian involvement gets exaggerated by people who are looking at Syria with a very sectarian agenda,” he said.
Damascus International Airport was shut down in December after rebels closed in on the outskirts of the airport perimeter and declared that all flights into and out of Syrian capital were legitimate military targets, on the grounds that Iran and Russia were flying in men and materiel to help Mr Al Assad’s war effort.
The airport has reopened since, although services have been cut back heavily, with carriers reluctant to fly into a war zone. Sporadic international flights continue between Syria and Iran, other Middle Eastern countries and eastern Europe.
Rebels, fighting against a renewed offensive by pro-regime units in the area, have kept up their efforts to attack the airport. Last Tuesday, their rockets struck the perimeter of the airport, according to opposition activists.