The United States will send extra military aircraft to Australia's tropical north this year, a U.S. Marine Corps spokesman said Wednesday, bolstering its military presence close to the disputed South China Sea.

The marines' deployment in the strategic city of Darwin, agreed in 2011, was a critical part of former U.S. President Barack Obama's "pivot" to Asia amid China's increased assertiveness in the region.

The move also cemented close ties with staunch ally Australia and gave the U.S. a foothold in the area.