The Iraqi security forces' seemingly inexorable push toward Mosul saw fighting begin in the city itself this week, but it is the work taking place in the skies above that has enabled the dramatic advance.

A relentless air operation using planes and drones from a dozen or so members of a US-led coalition against the Islamic State group over more than two years has conducted some 16,000 air strikes against the jihadists in Iraq and Syria -- two-thirds of them in Iraq.

Coalition planes have mostly conducted "deliberate" strikes in which they destroyed targets identified through intelligence or surveillance.

However, "now it's almost entirely dynamic strikes," a US military official told reporters during a recent visit to the Combined Air Operations Center (CAOC), where flights over Mosul and elsewhere are coordinated.

Dynamic strikes occur when ground units need air support against unexpected targets, such as an enemy fighting position or a car bomb being driven across the desert.

"The operation in Mosul is the number one priority right now," another US military official said.

Consider air activities in and around Mosul for just one day.

On Wednesday, coalition planes destroyed an IS headquarters building, five storage containers, two mortar systems, two fighting positions, a bomb-making factory, a supply cache, a sniper position, an observation post and a culvert.

The strikes also engaged two tactical units and damaged two tunnels, a bridge and a supply route, according to the coalition.

Air support is given to Iraqi security forces and Kurdish fighters, but not Shiite militiamen, and the Iraqi military has its own planes.

- Three bullet holes -

The coalition has enjoyed total air superiority in its anti-IS campaign. Although planes have come under sporadic small-arms fire, the most significant damage occurred when a C-130 cargo plane took three bullets to its tail.

Officials worry jihadists might seize "prestige" weapons such as surface-to-air missiles in the chaos of neighboring Syria, but so far that threat has not materialized.

Lacking military means, IS fighters are using civilians trapped in Mosul as human shields to prevent strikes.

The IS group has also experimented with rudimentary air attacks, using hobby planes and toy drones to carry explosives, and have tested a drone carrying a bottle containing some sort of chemical, a military official said.

Thanks to their air domination, coalition planes are constantly scouting Iraqi and Syrian skies in an operation that has cost almost $10 billion to date.

At any given time, 15-20 coalition aircraft -- mostly drones -- are circling Mosul, creating a green "donut" on radar screens that pinpoint each plane's location with a small icon. Russian planes, which operate in the same air space over Syria, are yellow.

The coalition has dropped more than 3,000 bombs on IS targets in the Mosul campaign, according to US Air Force Colonel John Dorrian.

"We're talking about hundreds of fighters (killed). We're talking about scores of vehicle-borne improvised explosive devices," he said Thursday.

"It's very impactful and it makes a big difference. We're very proud of that work."

AFP was among a small group of journalists allowed into the CAOC, whose location can be described only as a "military base in Southwest Asia."

The center is the primary hub for anti-IS air operations and coalition intel, where a never-ending stream of video from drones appears across eight huge monitors in the cavernous operations room.

One feed shows a small white pickup truck somewhere in Iraq. The drone's eye follows the vehicle into a compound, where a man loads something into the back.

The drone is 10,000 feet (3,000 meters) above but the footage is so clear he is seen tucking his loose white shirt into his pants. The man's fate is unclear; a military official said he probably was an IS fighter.

Even as the Mosul fight rages, officials are looking to Raqa in Syria as the next step in the campaign. Like it did in Mosul, the coalition has started hitting targets to "soften" areas in and around the city ahead of an assault.

The United States carries out most Iraq air operations, but Australia, Belgium, Canada, Denmark, France, Jordan, the Netherlands and Britain have also between them launched more than 3,000 sorties there.

Iraqi security forces approaching Mosul grew accustomed to continual coalition air cover. But as the advance split along multiple axes, some units complained they were not getting enough.

"Because they didn't have folks right over the top of them, they didn't think we were going to be able to respond to them, when in fact we were able to," Air Force Lieutenant General Jeffrey Harrigian said, noting that a jet traveling at 400 miles (645 kilometers) per hour can quickly cover the entire Mosul area.

"Each time our partners on the ground required an air asset, we were able to support them, and we will continue to do that."