Jim Michaels

USA TODAY

SCOTT AIR FORCE BASE, Illinois — The U.S. Air Force is increasing airdrops of weapons, ammunition and other equipment to a growing number of opposition forces closing in on Raqqa, the Islamic State’s de facto capital in Syria.

“Our expanded precision airdrop capability is helping ground forces take the offensive to (the Islamic State) and efforts to retake Raqqa,” said Gen. Carlton Everhart, commander of the Air Mobility Command, which is headquartered here.

The Air Force conducted 16 airdrop missions in Syria last year, including six in December.

The airdrops are essential in getting supplies to a force that doesn’t have extensive ground supply lines and is in nearly constant contact with the enemy, highlighting the complexity of supporting an irregular force operating in a hostile environment.

“In those instances airdrops are absolutely essential,” said Air Force Col. John Dorrian, a military spokesman in Baghdad.

The U.S.–led coalition is backing a force of about 45,000 fighters in Syria with airstrikes and teams of U.S. Special Forces, who are providing advice and training.

The force is becoming increasingly important as it places pressure on the Islamic State's most important stronghold in Syria. Over the border, Iraqi security forces are conducting an offensive in Mosul, the country's second-largest city and the militants' remaining stronghold in Iraq.

The Mosul battle is a more conventional operation where Iraq's army can use roads to get supplies.

The fight in Syria is being waged by tribal and other irregular forces. Airdrops are a lifeline to the forces there.

The airdrop missions have changed dramatically since previous wars, such as Vietnam, when pallets would be easily blown off target, sometimes landing within reach of the enemy.

Today, the bundles are guided onto landing zones using GPS technology and steerable parachutes. "We'll get it within 10 or 15 meters of the mark," Everhart said. The supplies range from small arms ammunition to vehicles.

The Air Force can drop supplies at night and vary where they are dropped to ensure militants are not able to seize U.S. equipment.

The Syrian Democratic Forces, which consists of Kurdish and Arab units, have been growing in size and have made advances toward Raqqa in recent months. The Pentagon said the fighters have liberated about 300 miles of territory from the Islamic State on its march toward the city.

The U.S.-led coalition is directly providing supplies only to the Arab contingent within the Syrian Democratic Forces, partly to avoid antagonizing Turkey, a key ally.

Initially, Kurds made up the bulk of the coalition-backed forces fighting the Islamic State in northern Syria. But U.S. advisers have made an effort to recruit more Arabs. Today, about one third of the 45,000 troops in the Syrian Democratic Forces are Arabs.

Diversifying the force is critical because of Turkish concerns about the growing influence of Kurds on their border and the need to build a more diverse force to liberate Raqqa, a mostly Arab city.

“We're looking for a force from the Raqqa environs and mostly Arabs because that is the ethnic composition of Raqqa,” said Lt. Gen. Stephen Townsend, commander of the U.S.–led task force fighting the Islamic State.