Incirlik airbase in southern Turkey has been a prized US facility since the depths of the Cold War but its strategic value has often been held hostage to discord between Ankara and Washington.

A landmark deal letting US warplanes use Incirlik to strike Daesh jihadists in neighbouring Syria marks the latest chapter in the base's long involvement in conflicts.

But its prime site near many of the world's trouble spots must be weighed against restrictions that NATO ally Turkey has frequently imposed, notably excluding the base from combat operations in Afghanistan and in the 2003 Iraq War.

"Incirlik has always been problematic," Michael Stephens of the RUSI Qatar research unit at the Royal United Services Institute in London told AFP.

"US officials will often tell you that the Turkey relationship has long been problematic -- but they are a NATO ally, like it or not, even if still a slightly recalcitrant one," said Stephens.

About 1,500 US military serve in the sprawling base as part of the US 39th Air Base Wing, which provides support to US and NATO aircraft and personnel in the region, the base's website says.

Lying close to the city of Adana just east of the Syrian border, Incirlik also provides air transport, communications and support services across the region, it says.

- 'Strike aircraft and drones' -

It took months of pressure from Washington for Turkey to finally give permission to use Incirlik as a base for raids against IS in Syria.

The deal foresees a no-fly zone along part of the Syrian border, spanning 90 kilometres (56 miles) between the Syrian towns of Marea and Jarabulus to the east, the Hurriyet daily said.

Where necessary, US-led coalition forces could conduct reconnaissance and assault operations in the area, Hurriyet said.

As a NATO member, Turkey has the right to restrict the use of the base to only alliance-linked activities.

The anti-IS raids, like the Iraq War, involve a non-NATO coalition.

"I wouldn't be surprised if you start seeing the full package at Incirlik now, I think you will be seeing ground strike aircraft, a lot of drones," said Stephens of RUSI.

"It reflects the growing importance of Syria as the main theatre of the conflict against Islamic State."

Stephens added though that Incirlik would likely only be used in combination with other US bases across the Middle East.

"There are plenty of other bases in the region that the US can operate from, but being 60 miles (100 kilometers) away from the contact zone, that is clearly where you want to be," he said.

- Cold War relic -

Incirlik was built in 1951 in the coldest days of the Cold War as an American bastion ideally placed near both the Soviet Union and the turbulent Middle East.

One of its main roles was a base for spy flights by high-altitude U-2 aircraft over Soviet territory, although that stopped after US pilot Gary Powers was shot down in 1960, handing Moscow a propaganda coup.

In 1975, Turkey took back control of Incirlik after Washington cut military assistance to Ankara because it used US-supplied equipment during the invasion of Cyprus. The base returned to US control in 1980.

After the Cold War, US and British warplanes used the base to patrol the no-fly zone over northern Iraq between the end of the 1991 Gulf War and the invasion of Iraq in 2003.

Following the September 11, 2001 attacks on the United States, Incirlik became the main hub for air support missions for the US-led invasion of Afghanistan to topple the Taliban regime, including airlifts and air refuelling missions.

But Turkey refused to let the US launch air raids from Incirlik during the 2003 war.

It did however later allow cargo flights. Incirlik subsequently became the main transport hub from Iraq for US forces.
Source: AFP