U.S. Marines walk after disembarking a plane after they arrived for the sixth annual Marines' deployment at Darwin in northern Australia, April 18, 2017. REUTERS/Tom Westbrook

SYDNEY (Reuters) - Three hundred U.S. Marines flew out of Australia on Sunday as their troop rotation came to an end, the first of a 1,250-strong force to leave the Pacific nation after being stationed for six months in the country’s far north.

The sixth rotational group was the most advanced contingent to be sent since the program began after former U.S. President Barack Obama agreed to send troops in 2011 as part of a military pivot to the Asia-Pacific region.

For the first time the rotation included an aviation combat section of 13 aircraft, including four MV-22 Osprey tilt-rotor aircraft, according to Australia’s Department of Defence.

Australian and U.S. troops also practiced disaster relief and humanitarian assistance as a combined force in a remote location for the first time.

The rotational forces are stationed in Darwin, state capital of the Northern Territory, which is strategically located on the coastal doorstep of Indonesia, East Timor and Papua New Guinea.

During the six-month deployment, the U.S. troops participated in 12 joint exercises with the Australian Defence Force, giving them an opportunity to interact with a range of countries including China, Indonesia, Japan, New Zealand, Malaysia, the Philippines, Singapore, Thailand and Vietnam.

“Australia’s alliance with the United States is our most important defense relationship and remains central to Australia’s security,” Captain Bryan Parker, deputy commander of Australia’s Northern Command, said in a statement.

“The presence of the U.S. Marines across the Northern Territory plays a vital role in underwriting security and stability across the Indo-Pacific,” he said, adding that force posture initiatives were essential to preserving stability over the coming decades.

Marine rotation numbers were originally expected to reach 2,500 by 2017 but so far have not risen above 1,250, according to ABC News reports.