Washington (AFP) - US military aircraft have delivered food, water and ammunition to besieged Iraqi troops battling Islamic State militants, the Pentagon said Saturday, as the coalition pursued its air assault in Iraq and Syria.

The move marked the first time that coalition aircraft airdropped supplies to Iraqi government forces. Similar drops were previously part of humanitarian missions for civilians, such as Yazidis and Turkmen.

The airdrops of vital supplies to troops in northern Iraq on Friday and Saturday came at Baghdad's request, US Central Command said in a statement.

"US military forces conducted multiple airdrops in Iraq in the vicinity of Baiji... to resupply Iraqi security forces operating in the area," it added.

In total, 36 bundles containing 7,328 halal meals, 2,065 gallons of water and 16,000 pounds of ammunition were dropped.

While Baiji remains under Baghdad's control, nearby towns "are contested as ISIL continues to conduct operations in the area," the statement said, using another acronym to identify the IS group.

The embattled Iraqi army is facing increasing pressure from IS jihadists in several hotspots.

The airdrops come after US Defense Secretary Chuck Hagel described the campaign against IS in Iraq as "difficult," particularly in the western province of Anbar.

"Anbar province is in trouble. We know that," he said on Friday during a visit to Colombia.

"This is a difficult effort. It is going to take time. It won't be easy. So, yes, there is a lot of uncertainty in Anbar right now."

The US-led coalition also launched fresh air strikes in Iraq and Syria on Friday and Saturday, Central Command said.

Bomber and fighter jets conducted six air strikes in Syria, including four in the battleground town of Kobane near the Turkish border.

In Iraq, three air strikes were carried out by attack and fighter jets -- one in Tal Afar in the north and two in Hit in Anbar province -- with the help of Dutch aircraft.

A US-led coalition has been conducting air strikes in Iraq since early August and in Syria since mid-September.

IS fighters have beheaded four Western hostages and seized huge swaths of territory in Iraq and Syria.